Stefan Nemanjić was the King of Serbia from 1217 until his death in 1228. He was the first Rascian king; due to his transformation of the Serbian Grand Principality into the Kingdom of Serbia and the assistance he provided his brother Saint Sava in establishing the Serbian Orthodox Church, he is regarded one of the most important members of the Nemanjić dynasty.

In Byzantium, Sava managed to secure autocephaly (independence) for the Serbian Church and became the first Serbian archbishop in 1219. In the same year Sava published the first constitution in Serbia — St. Sava's Nomocanon. The Nomocanon was a compilation of Civil law, based on Roman Law, and Canon law, based on Ecumenical Councils. Its basic purpose was to organize the functions of the young Serbian kingdom and the Serbian church. Thus the Serbs acquired both political and religious independence.

In 1220, grand assembly of the realm was held in Žiča, were Stefan was crowned by the Orthodox ritual and coronation was performed by archbishop Sava. That act served as a precedent for all their successors: all Serbian kings of the Nemanjić dynasty were crowned in Žiča, by Serbian archbishops.

In the time of Tsar Samuel of Bulgaria, Bogomilism spread into Serbia and Bosnia. The most active area became west Bosnia, centered on the valley of the River Bosna. In the province of Hum (modern Herzegovina) the Bogomils were also strong, in the cities of Split and Trogir Bogomils were numerous but later they took refuge in Bosnia. Providing refuge to those labeled heretics, including Bogomils, was a recurrent pretext for Hungarian rulers to declare crusades against Bosnia and extend their influence in the region. A first Hungarian complaint to the Pope was averted by the public abjuration of the Bosnian ruler Ban Kulin, a close relative of Stefan Nemanja, in 1203.

A second Hungarian crusade against Bosnia on the pretext of Bogomil heresy was launched in 1225, but failed.

King Stefan I, who had become ill, took monastic vows and died in 1227. Radoslav who was the eldest son succeeded as King, crowned at Žiča by Archbishop Sava, his uncle and the first Archbishop (since 1219) of the autocephalous Serbian Orthodox Church. The younger sons, Vladislav and Uroš I, received appanages. The youngest of the brothers, Sava II, was appointed Bishop of Hum shortly thereafter, later serving as Archbishop of Serbia (1263-1270). The Church and state were thus dominated by the same family and the ties between the two as well as the family's role within the Church continued.

According to biographer and monk Theodosius, Radoslav was a good ruler at first, but then fell under the influence of his wife, who was the daughter of the ruler of Epirus and Thessalonica, Theodore Komnenos Doukas (r. 1215–1230). Radoslav was most likely not beloved by the Serbian nobility due to this Greek influence.

Serbian medieval biographers noted that the nobility had left the support of Radoslav and stood itself behind the younger Vladislav. Radoslav fled the country between 1 September 1233 and 4 February 1234, and was unable to regain the kingdom, but eventually returned as a monk.

During Vladislav's reign, Serbia was politically aligned with Bulgaria at the time, since Vladislav was married to Beloslava, the daughter of Ivan Asen II. Vladislav secured Hum, a maritime province under attack by Hungarian crusaders. After the death of Ivan Asen II, there was unrest in Serbia. The Mongols, led by Kadan, invaded Hungary and devastated the Balkans, at which time the Serbian nobility rose up against Vladislav. In 1243, he abdicated in favour of his younger brother, but remained the governor of Zeta.

The Bulgarians lost Braničevo and Belgrade to Hungary in the late 1230s, and Hungarian crusaders fought in Bosnia between 1235 and 1241. Serbia was never directly attacked by the Hungarians. However, the Hungarian crusaders did directly threaten Serbian Hum; they may have even occupied parts of it. In 1237, Coloman of Galicia-Lodomeria attacked Hum, but it is unclear whether they attacked Serbian Hum (Eastern), or western Hum, between the Neretva and Cetina rivers, where the Serbs held no territory at the time. The northern part, which was held by Vladislav's relative, Toljen II, fell quickly, but Vladislav dispatched an army to regain the region. The crusaders were pushed to the border, and Vladislav pursued them as far as the Cetina River, but there were no major encounters. After the incident, the Serbs asserted their possession of the Hum region, and Vladislav added "Hum" to his title.

In 1235, Vladislav signed a treaty regarding trading privileges with Giovanni Dandolo, a representative of Ragusa. The treaty gave Ragusa trading privileges under the condition that Ragusa would never allow any preparation of rebellion against Serbia on their territory, as Ragusa had helped Radoslav upon his exile.

In the winter of 1241, the Mongols crossed the Danube and entered western Hungary; Béla IV could not manage to organize any resistance. All of Croatia was burned, and Kadan and Batu Khan (the grandson of Genghis Khan) looked for Béla IV, who was in Split at the time; Béla soon moved to Trogir, as Split was not safe. The Mongols did not attack Split, but instead unsuccessfully attacked Klis, where they had heard that Béla IV was hiding. Béla then fled to the island of Rab. The Mongols attempted to conquer the island, but their forces were hurt in naval battles; they were also forced to hurry back home to choose the new Khan after the death of Ogatay. While returning home, they crossed and devastated Serbia, Bosnia and Bulgaria.

When the Mongol commander Kadan withdrew from the invasion of Hungary, he entered Bosnia in late March or early April 1242. Although nominally under Hungarian suzerainty, part of Bosnia had been occupied by Hungarian crusaders opposed to the Bosnian Church while the remainder was under the control of Ban Matej Ninoslav. The passing-through of the Mongols forced the Hungarians to evacuate the territory and allowed Ninoslav to resume control of the whole of Bosnia.

Continuing south, the Mongols entered the Serbian region of Zeta (roughly Montenegro and northern Albania). According to Archdeacon Thomas of Split, they inflicted minimal damage on independent Dubrovnik, which was too strong to take. In Zeta, however, the forces of Kadan attacked Kotor, razed to the ground Svač and Drisht and probably also destroyed Sapë, which was only rebuilt several decades later. In Thomas's words, the Mongols left behind in Zeta "nobody to piss against a wall".The city of Ulcinj may have been spared because of an agreement reached with Dubrovnik in April.

According to Thomas of Split, a contemporary and partial eyewitness, the Mongols "overran all of Serbia and came to Bulgaria". Another contemporary, the archdeacon Roger of Várad from Hungarian Transylvania, notes that "Kadan destroyed Bosnia and the kingdom of Rascia and then crossed into Bulgaria". This is all that is known about the invasion of Serbia proper (Rascia) from literary sources. The raiding and looting in Serbia was over by late spring, when the tumens had moved on to Bulgaria.

In spring 1243, an uprising ousted Vladislav; Stefan Uroš I, his third brother, was put on the throne. Scholars have argued that Bulgarian influence had been strong and unpopular, causing opposition that led to Vladislav's deposition after the death of Asen.

The revolting nobility had chosen Uroš as their candidate for king; from 1242 to spring 1243, a war for the throne was fought, which ended with Vladislav being forced to give up the crown in favour of Uroš. It seems that Uroš captured Vladislav and held him in prison. The hostilities did not last long, and the brothers quickly settled. Uroš was courteous towards Vladislav, gave him the administration of Zeta, with residence in Skadar.

Stefan Uroš I, known as Uroš the Great (Урош Велики) was the King of Serbia from 1243 to 1276, succeeding his brother Stefan Vladislav. He was one of the most important rulers in Serbian history.

The situation in Europe and in the Balkans were quite favorable for Serbia, which he very cleverly used for his benefit. During his reign Serbia significantly strengthened itself and progressed in every way. Uroš correctly determined the direction in political pretensions through penetrating the south in Macedonia and conflict with Hungary in Podunavlje. The land was politically and militarily prepared for serious politics and definitive fortification of Serbia and the Serb people in the Vardar valley and the middle Podunavlje.

Apart from this, Uroš also correctly determined the direction of Serbian trade politics, as he on several occasions in his fight against the Republic of Ragusa wanted to eliminate Ragusan brokerage and exploitation in his state.

In foreign policy Uroš skillfully used to his advantage the conflict between the Despotate of Epirus and the Empire of Nicaea, two Greek states, both of which sought to inherit the Byzantine Empire and take Constantinople from the Latin Empire. But when the Latin Empire fell, and Emperor Michael Palaiologos of Nicaea took Constantinople, Uroš began to coalite with his wife's cousin, Charles of Anjou, who wanted to recapture Constantinople, and through that alliance take as much Byzantine land as possible.

Under Stefan Uroš I, Serbia became a significant power in the Balkans, partly due to economic development through opening of mines. Uroš was the first to begin exploiting the mines, which would later become one of the main sources of material wealth and power of the Serbian state in the Middle Ages.

The mines were developed by the "Sasi" (Saxons), who were experienced in the extracting of ore. Their settlements, located by the mines, had privileged status – they lived under their own laws and were allowed to adhere to Catholicism and build their churches. Important mines were located at Novo Brdo, Brskovo and Rudnik. As a first result of the opening of mines came the forging of Serbian coins, which were first minted on the Venetian model.

Economic prosperity was also fostered by the related intensification of trade with the Dalmatian cities of Dubrovnik and Kotor. The increase in the mining of silver and in trade naturally led to the introduction of larger quantities of royal coinage, modeled after the Venetian standard.

In 1252–1253, Uroš I was at war with the Republic of Ragusa, which bordered the Hum, which was held by his kinsman Radoslav Andrijić. Radoslav swore to fight Ragusa as long as it was in conflict with Serbia, at the same time boasting relations with Béla IV of Hungary. Ragusa took up an alliance with Bulgaria. Peace was ensured in a charter dated May 22, 1254, and the crisis ended.

During the second half of the 1260s a new war broke out with Ragusa, which was secretly favored by the Serbian queen. A treaty was signed in 1268, specifying the amount of protection money that Dubrovnik was expected to supply annually to the Serbian king. The arrangement remained largely unbroken for the next century.

In 1268 the Serbian king invaded the Hungarian possessions south of the Danube in Mačva, what is now western central Serbia. In spite of some initial success, Stefan Uroš was captured by the Hungarians and forced to purchase his release. A peace treaty was signed between the two kingdoms, and Stefan Uroš's son Stefan Dragutin of Serbia was married to Catherine, the daughter of the future king Stephen V of Hungary.

By the end of his reign, Stefan Uroš apparently succeeded in suppressing the autonomy of Zahumlje, where the local princes became virtually indistinguishable from the rest of the nobility. In his effort to achieve centralization, the king appears to have alienated his eldest son by refusing to grant him an appanage. The conflict between father and son exacerbated, and the king apparently considered making his younger son, the future Stefan Milutin, his heir.

Worried about the inheritance and his very life, Stefan Dragutin finally demanded the throne in 1276. When Stefan Uroš refused, Dragutin rebelled and received help from his Hungarian relatives. The allies defeated the Serbian king and Stefan Uroš was forced to abdicate and retire to an unidentified monastery in Hum where he died a year or two later. His remains were later moved to his monastic foundation of Sopoćani.

Dragutin administered his realm independently of his brother. He supported the Franciscans' missions in Bosnia and allowed the establishment of a Catholic see in Belgrade.

Stefan Dragutin was King of Serbia from 1276 to 1282. From 1282, he ruled a separate kingdom which included northern Serbia, and (from 1284) the neighboring Hungarian banates (or border provinces), for which he was unofficially styled "King of Syrmia".

Dragutin abandoned Uroš I's centralizing policy and ceded large territories to his mother in appanage. After a riding accident, he abdicated in favor of his brother Milutin in 1282, but retained the northern regions of Serbia along the Hungarian border. Two years later, his brother-in-law, Ladislaus IV of Hungary, granted him three banates—Mačva (or Sirmia ulterior), Usora and Soli. He was the first Serbian monarch to rule Belgrade. With his brother's support, he also occupied the Banate of Braničevo in 1284 or 1285.

In theory, Dragutin was a vassal both to his brother (for his Serbian territories), and to the Hungarian monarchs (for the four banates), but in practice he ruled his realm as an independent ruler from the 1290s. His conflicts with Milutin developed into open war in 1301, and he frequently raided the neighboring Hungarian lords from 1307.

Dragutin fell off his horse and broke his leg in early 1282. His injury was so severe a council was called in Deževo to make decisions about governing Serbia. At the council, Dragutin abdicated in favor of Milutin, but the circumstances of his abdication are uncertain. Decades later, Dragutin recounted that he had already come into conflict with Milutin, and that he had ceded the government to Milutin only provisionally, until he recovered. Archbishop Danilo II wrote that Dragutin abdicated because he regarded the riding accident as God's punishment for his acts against his father, but the Archbishop also referred to unspecified "serious troubles" that contributed to Dragutin's decision. The Byzantine historian, George Pachymeres, was informed that Dragutin's abdication had been definitive, but Pachymeres also mentioned an agreement between the two brothers that secured the right of Dragutin's (unnamed) son to succeed Milutin.

Stefan Uroš II Milutin was the King of Serbia between 1282–1321, a member of the Nemanjić dynasty. He was one of the most powerful rulers of Serbia in the Middle Ages. Milutin is credited with strongly resisting the efforts of Byzantine Emperor Michael VIII Palaiologos to impose Roman Catholicism on the Balkans after the Union of Lyons in 1274.

During his regin, Serbian economic power grew rapidly, mostly due to the development of mining. He founded Novo Brdo, which became an internationally important silver mining site. As most of the Nemanjić monarchs, he was proclaimed a saint by the Serbian Orthodox Church with a feast day on October 30.

In 1282, the Serbian king Stefan Milutin invaded northern Macedonia, then a part of the Byzantine Empire. The Emperor Michael VIII was distracted at the time by his conflict with the Despot John I of Thessaly, however, and called upon Nogai Khan to provide him with troops to attack Thessaly. Nogai sent 4,000 cavalrymen, who arrived in Thrace in October. On 11 December, however, Michael VIII died. His son, Andronikos II, did not wish to pursue the attack on Thessaly, so instead sent the Mongols across the Danube to, in the words of Nicephoros Gregoras, "weaken e Serbs] and then to return with plunder over the Danube". The army, which included Byzantine auxiliaries, was placed under the command of Michael Tarchaneiotes.

In early 1283, the Byzantino-Mongol force crossed the Danube and plundered as far as Lipljan and Prizren. A Mongol detachment attempted to cross the river Drim and was defeated by the Serbs. Their leader, named Blackhead according to the Serbian archbishop Danilo II, was captured and beheaded. A majority of the Mongols must have returned, however, since Gregoras calls the entire mission a success.

The Serbs were not weakened or deterred by Andronikos' operation. In the fall of 1283, Milutin invaded Macedonia again, penetrating all the way to Kavala on the Aegean coast.

According to the Danilo, in the early 1280s the Bulgarian princes Darman and Kudelin were harassing the Hungarian banate of Macsó (Mačva) with the help of their Tatar (Mongol) and Cuman allies. In late 1284, King Ladislaus IV of Hungary gave Macsó, including Belgrade and some territory in northern Bosnia, to the deposed Serbian king Dragutin, who in 1282 had set up his own kingdom north of the Western Morava. In 1285, Dragutin allied with Hungary and attacked Darman and Kudelin. This attack was repulsed and the Bulgarians with their Cuman and Tatar mercenaries ravaged Dragutin's lands. They occupied Macsó and Dragutin himself was forced to flee to the court of Milutin.

According to a letter of King Andrew III of Hungary, in the winter of 1291–92 the region of Macsó (which was under Dragutin) was attacked by Mongols and he sent an army there to defend it. This attack on Macsó could have come from Bulgarian or Serbian territory, most likely that of Darman and Kudelin. Later in 1292, Dragutin allied with Milutin and together they defeated Darman and Kudelin. Dragutin annexed the regions of Braničevo and Kučevo from them and they fled across the Danube to Mongol territory.

Dragutin sought help from Milutin and the two brothers met in Mačkovac. After they joined their forces and defeated Darman and Kudelin, Dragutin seized Braničevo in 1291 or 1292. The new Hungarian monarch, Andrew III, also supported their military action, but Andrew's weak position in Hungary enabled Dragutin to strengthen his independence.

Following the annexation of Braničevo, the borders of Dragutin's Serbia were brought up to the territory of Shishman. He may have been an erstwhile ally or even vassal of Darman and Kudelin; he was certainly a vassal of the Golden Horde, which may have even installed him in Vidin. In 1292, he "gathered thrice-cursed Tatar heretics and his own soldiers", in the words of Danilo, and invaded Milutin's Serbia.

Shishman's army contained a large number of Mongols and, unlike in the army of Darman and Kudelin, these were not mercenaries. His invasion was no more than a plundering raid, but it plundered deep into Serbian territory and caused major devastation, including the burning of the monastery of Žiča. He was defeated near Ždrelo and then retreated. In response, Milutin invaded Shishman's territory and took Vidin, forcing Shishman to flee across the Danube to the territory of the Golden Horde. Soon after, Shishman was re-installed in Vidin under Serbian suzerainty, probably at the insistence of the Mongols. This probably happened in 1292. To seal the alliance, Shishman married the daughter of a Serbian župan named Dragoš, and his son Michael married Milutin's daughter Anna.

Despite their obvious diplomatic victory, since Shishman was back on the throne in Vidin, the Mongols of the Golden Horde clearly regarded Milutin's successes as at their expense. According to Danilo, he "began preparations to strike lutin with heathen forces, wanting to seize his lands". Warned in advance of Nogai's preparations, Milutin sent an embassy to the khan's court, where evidently he offered to accept Mongol overlordship. Danilo records that afterwards he sent his son Stefan Dečanski, the future king of Serbia, and "the high nobles of Serbian lands" to Nogai's court. These could only have been hostages and possibly a small military contingent. In any case, they were symbols of Serbian submission. This must have taken place between 1293 and 1294. Dečanski remained a hostage until 1297.

Tensions between the two brothers grew rapidly, most probably because Milutin wanted to secure the succession in Serbia for his own sons. In 1301, open war broke out and Milutin occupied Rudnik after taking it from Dragutin. According to Ragusan reports, a peace treaty was made in late 1302, but Dragutin's troops or allies pillaged Milutin's silver mines at Brskovo in 1303. The armed conflict lasted for more than a decade, but its details are unknown. The parties allegedly avoided fighting pitched battles and Dragutin kept his realm almost intact, although income from the silver mines enabled Milutin to hire mercenaries.

Upon Stefan Dragutin's death in 1316, Milutin conquered most of his lands including Belgrade. That was not acceptable for king Charles I of Hungary, who started to seek allies against Serbia, including those among Albanian nobles, who were also receiving support from Pope John XXII. Milutin started to persecute Catholics which led to the crusade started by Pope John XXII.

In 1318, there was an open revolt of Albanian nobles against the rule of Stefan Milutin, which is sometimes credited to be incited by Prince Philip I of Taranto and Pope John XXII in order to weaken Stefan Milutin's rule. Milutin suppressed the rebels without much difficulty. In 1319, Charles I of Hungary regained control over Belgrade and the region of Mačva while Milutin held control in Braničevo.

Milutin became ill and died on 29 October 1321, leaving no formal instruction regarding his inheritance. Konstantin was crowned King in Zeta, but civil war broke out immediately as both Dečanski and his cousin, Stefan Vladislav II, claimed the throne. Dečanski revealed that his eyesight was still intact, claiming a miracle, and the populace rallied behind him believing the restoration of his sight to be a sign from God. On 6 January 1322, the archbishop of Serbia, Nicodemus, crowned Dečanski king and his son, Stefan Dušan, the young king. Dečanski later granted Zeta to Dušan as a fief, indicating his intention for Dušan to be his heir. According to one account, Dečanski offered to split the realm with Konstantin, who refused. Dečanski then invaded Zeta, and Konstantin was defeated and killed.

In the meantime, Vladislav II had been released from prison upon Milutin's death and recovered the throne of Syrmia, which his father had established in northern Serbia. Vladislav also claimed the throne of Serbia upon Milutin's death and mobilized local support from Rudnik, a former possession of Vladislav's father. Also supported by Hungarians, Bulgarians, and Bosnians, Vladislav consolidated control over Syrmia and prepared for battle with Dečanski.

In 1323, war broke out between Dečanski and Vladislav. In autumn, Vladislav still held Rudnik, but by the end of 1323, the market of Rudnik was held by officials of Dečanski, and Vladislav seems to have fled further north. Some of Vladislav's supporters from Rudnik, led by Ragusan merchant Menčet, took refuge in the nearby Ostrovica fortress, where they resisted Dečanski's troops. Dečanski sent envoys to Dubrovnik (Ragusa), to protest the support of Vladislav. Dubrovnik rejected Dečanski's complaint, claiming Ostrovica was held by Serbs. Dečanski was not satisfied, and in 1324 he rounded up all the Ragusan merchants he could find, confiscated their property, and held them captive. By year's end, Rudnik was restored to Dečanski, who released the merchants and returned their property. Vladislav was defeated in battle in late 1324, and fled to Hungary, that was holding Belgrade since 1319.

The growing power of the Serbian Kingdom from the late 13th century raised serious concerns in the traditional Balkan powers Bulgaria and Byzantine Empire which agreed for joint military actions against Serbia in 1327. Three years later the bulk of the Bulgarian and Serbian armies clashed at Velbazhd and the Bulgarians were caught by surprise. Serbian victory shaped the balance of power in Balkans for the next two decades. The Bulgarians did not lose territory after the battle but were unable to stop the Serbian advance towards Macedonia. Serbia managed to conquer Macedonia and parts of Thessaly and Epirus reaching its greatest territorial extent ever. Their new king Stefan Dušan was crowned Emperor with support from Bulgarian Patriarch Symeon in 1346.

Stefan Uroš IV Dušan was the King of Serbia from 8 September 1331 and Tsar (or Emperor) and autocrat of the Serbs and Greeks (or Romans) from 16 April 1346 until his death in 1355.

Dušan conquered a large part of southeast Europe, becoming one of the most powerful monarchs of the era. Under Dušan's rule, Serbia was the major power in the Balkans, and an Eastern Orthodox multi-ethnic and multi-lingual empire that stretched from the Danube in the north to the Gulf of Corinth in the south, with its capital in Skopje. He enacted the constitution of the Serbian Empire, known as Dušan's Code, perhaps the most important literary work of medieval Serbia. Dušan promoted the Serbian Church from an archbishopric to a patriarchate, finished the construction of the Visoki Dečani monastery (now a UNESCO site), and founded the Monastery of the Holy Archangels, among others. Under his rule, Serbia reached its territorial, political, economic, and cultural peak.

Dušan died in 1355, seen as the end of resistance against the advancing Ottoman Empire and the subsequent fall of the Eastern Orthodox Church in the region.

Dušan began to fight against the Byzantine Empire in 1334, and warfare continued with interruptions of various duration until his death in 1355. Twice he became involved in larger conflicts with the Hungarians, but these clashes were mostly defensive.

Dušan's armies were initially defeated by Charles I of Hungary's 80,000-strong royal armies in Šumadija in 1336. As the Hungarians advanced south towards a hostile terrain, Dušan's cavalry launched several attacks in the narrow open fields, resulting in a rout of Hungarian troops, which retreated to the north of Danube. Charles I was wounded by an arrow but survived. As a result, the Hungarians lost Mačva and Belgrade.

The Byzantine civil war of 1341–1347, sometimes referred to as the Second Palaiologan Civil War, was a conflict that broke out in the Byzantine Empire after the death of Andronikos III Palaiologos over the guardianship of his nine-year-old son and heir, John V Palaiologos. It pitted on the one hand Andronikos III's chief minister, John VI Kantakouzenos, and on the other a regency headed by the Empress-Dowager Anna of Savoy, the Patriarch of Constantinople John XIV Kalekas, and the megas doux Alexios Apokaukos. The war polarized Byzantine society along class lines, with the aristocracy backing Kantakouzenos and the lower and middle classes supporting the regency. To a lesser extent, the conflict acquired religious overtones; Byzantium was embroiled in the Hesychast controversy, and adherence to the mystical doctrine of Hesychasm was often equated with support for Kantakouzenos.

Dušan's systematic offensive began in 1342, and in the end he conquered all Byzantine territories in the western Balkans as far as Kavala, except for the Peloponnesus and Thessaloniki, which he could not besiege due to his small fleet. There has been speculation that Dušan's ultimate goal was no less than to conquer Constantinople and replace the declining Byzantine Empire with a united Orthodox Greco-Serbian Empire under his control. In May 1344, his commander Preljub was stopped at Stephaniana by a Turkic force of 3,100. The Turks won the battle, but the victory was not enough to thwart the Serbian conquest of Macedonia. Faced with Dušan's aggression, the Byzantines sought allies in the Ottoman Turks, whom they brought into Europe for the first time.

On 16 April 1346 (Easter), Dušan convoked a huge assembly at Skopje, attended by the Serbian Archbishop Joanikije II, the Archbishop of Ochrid Nikolas I, the Bulgarian Patriarch Simeon, and various religious leaders of Mount Athos. The assembly and clerics agreed upon, and then ceremonially performed, the raising of the autocephalous Serbian Archbishopric to the status of Serbian Patriarchate. The Archbishop from then on was titled Serbian Patriarch, although some documents called him Patriarch of Serbs and Romans, with the seat at the Monastery of Peć. The first Serbian Patriarch Joanikije II solemnly crowned Dušan as "Emperor and autocrat of Serbs and Romans". Dušan had his son Uroš crowned King of Serbs and Greeks, giving him nominal rule over the Serbian lands, and although Dušan was governing the whole state, he had special responsibility for the Eastern Roman lands.

In 1347, Dušan conquered Epirus, Aetolia and Acarnania, appointing his half-brother, despot Simeon Uroš as governor of those provinces. In 1348, Dušan also conquered Thessaly, appointing Preljub as governor. In eastern regions of Macedonia, he appointed Vojihna as governor of Drama. Once Dušan conquered Byzantine possessions in western regions, he sought to obtain Constantinople.

To acquire the city, he needed a fleet. Knowing that fleets of southern Serbian Dalmatian towns were not strong enough to overcome Constantinople, he opened negotiations with Venice, with which he maintained fairly good relations. Venice feared a reduction of privileges in the Empire if Serbs became the masters of Constantinople over the weakened Byzantines. But if the Venetians had allied with Serbia, Dushan would have examined existing privileges. Once he became master of all Byzantine lands (especially Thessalonika and Constantinople) the Venetians would have gained privileges. But Venice chose to avoid a military alliance. While Dušan sought Venetian aid against Byzantium, the Venetians sought Serbian support in the struggle against the Hungarians over Dalmatia. When sensing that Serbian aid would result in a Venetian obligation to Serbia, Venice politely turned down Dušan's offers of help.

The Black Death (also known as the Pestilence, the Great Mortality or simply, the Plague) was a bubonic plague pandemic occurring in Afro-Eurasia from 1346 to 1353. It is the most fatal pandemic recorded in human history, causing the death of 75–200 million people in Eurasia and North Africa, peaking in Europe from 1347 to 1351. Bubonic plague is caused by the bacterium Yersinia pestis spread by fleas, but it can also take a secondary form where it is spread person-to-person contact via aerosols causing septicaemic or pneumonic plagues.

In 1350, Dušan attacked Bosnia, seeking to regain the previously lost land of Hum and stop raids on his tributaries at Konavle. Venice sought a settlement between the two but failed. In October he invaded Hum, with an army said to be of 80,000 men, and successfully occupied part of the disputed territory. According to Orbini, Dušan had secretly been in contact with various Bosnian nobles, offering them bribes for support. Many nobles, chiefly of Hum, were ready to betray the Ban, such as the Nikolić family, which was kin to the Nemanjić dynasty. The Bosnian Ban avoided any major confrontation and did not meet Dušan in battle; he instead retired to the mountains and made small hit-and-run actions. Most of Bosnia's fortresses held out, but some nobles submitted to Dušan. The Serbs ravaged much of the countryside. With one army they reached Duvno and Cetina; another reached Krka, on which lay Knin (modern Croatia); and another took Imotski and Novi, where they left garrisons and entered Hum. From this position of strength, Dušan tried to negotiate peace with the Ban, sealing it by the marriage of Dušan's son Uroš with Stephen's daughter Elizabeth, who would receive Hum as her dowry – restoring it to Serbia. The Ban was not willing to consider this proposal.

Saint Stefan Uroš V, known as Uroš the Weak, was the second Emperor (Tsar) of the Serbian Empire (1355–1371), and before that he was Serbian King and co-ruler (since 1346) with his father, Emperor Stefan Dušan.

The account of the contemporary John VI Kantakouzenos describes a descent of the Serbian Empire into disintegration soon after death of Uroš' father and his accession. Further the general disorder long with the powerlessness of the center represents the situation that arose much later in Uroš's reign. According to Mihaljčić, during the initial years of his rule the threats to the territorial integrity of Uroš's empire in the south came mainly from external attacks.

In following years, the Serbian Empire gradually fragmented into a conglomeration of principalities, some of which did not even nominally acknowledge Uroš's rule. His position was not helped by his mother Helena, who started to rule autonomously from Serres in alliance with Jovan Uglješa. A similarly autonomous posture was assumed by the Dejanović family, the Balšić family, Nikola Altomanović. By 1365, the most powerful Serbian nobleman became Uglješa's brother Vukašin Mrnjavčević who became co-ruler with Emperor Uroš and was granted the title of Serbian King. By 1369, as Uroš was childless, Vukašin designated his eldest son Prince Marko as heir to the throne, with the title of "young king".

In the summer of 1371, Vukašin marched to Zeta, to support his relative Đurađ Balšić in his war against Nikola Altomanović. His army was in Skadar, waiting for naval support from the Republic of Ragusa. Uglješa received information that the majority of Ottoman forces left Europe and marched to Anatolia. He decided it was a good time to execute his offensive plans and asked Vukašin for help. Vukašin left Skadar with his army and joined Uglješa. They marched against Adrianople.

The Serbian army numbered 50,000–70,000 men. Despot Uglješa wanted to make a surprise attack on the Ottomans in their capital city, Edirne, while Murad I was in Asia Minor. The Ottoman army was much smaller, Byzantine Greek scholar Laonikos Chalkokondyles and different sources give the number of 800 up to 4,000 men, but due to superior tactics, by conducting a night raid on the Serbian camp, Şâhin Paşa was able to defeat the Serbian army and kill King Vukašin and despot Uglješa. Thousands of Serbs were killed, and thousands drowned in the Maritsa river when they tried to flee. After the battle, the Maritsa ran scarlet with blood. Parts of Macedonia and Thrace fell under Ottoman power after this battle. 

Stefan Uroš V died childless in December 1371, after much of the Serbian nobility had been destroyed by the Turks in the Battle of Maritsa earlier that year. The exact cause of his death at a relatively young age remains unknown. Vukašin's son Prince Marko inherited his father's royal title, but real power in northern Serbia was held by Lazar Hrebeljanović. The latter did not assume the imperial or royal titles (associated with the Nemanjić dynasty), and in 1377 accepted King Tvrtko I of Bosnia (a maternal grandson of Stefan Dragutin) as titular king of Serbia. Serbia proper became a vassal of the Ottomans in 1390, but remained effectively ruled by the Lazarević family and then by their Branković successors until the fall of Smederevo in 1459.

Today, Stefan Uroš V is viewed mostly in contrast to his able and strong-willed father, as a lacking and indecisive ruler, unable to keep the Serbian nobility under his control, whose weak and unassertive personality greatly contributed to the fall of the Empire and the eventual destruction of the Serbian state by the Ottomans.