The causes of the First Crusade are widely debated among historians. By the start of the 11th century Europe, the influence of the papacy had declined to that of little more than a localised bishopric.

Compared with Western Europe, the Byzantine Empire and the Islamic world were historic centres of wealth, culture and military power. The first waves of Turkic migration into the Middle East enjoined Arab and Turkic history from the 9th century. The status quo in Western Asia was challenged by later waves of Turkish migration, particularly the arrival of the Seljuk Turks in the 10th century.

Byzantine emperor Alexios I Komnenos, worried about the advances of the Seljuqs in the aftermath of the Battle of Manzikert, who had reached as far west as Nicaea, sent envoys to the Council of Piacenza in March 1095 to ask Pope Urban II for aid against the invading Turks.

In July 1095, Urban turned to his homeland of France to recruit men for the expedition. His travels there culminated in the ten-day Council of Clermont, where on Tuesday 27 November he gave an impassioned sermon to a large audience of French nobles and clergy. According to one version of the speech, the enthusiastic crowd responded with cries of Deus vult! ("God wills it!").

Several groups organically formed and headed their own crusader 'armies' (or mob) and headed towards the holy land by way of the Balkans. A charismatic monk and powerful orator named Peter the Hermit of Amiens was the spiritual leader of the movement. Peter gathered his army at Cologne on 12 April 1096. There were also many knights among the peasants, including Walter Sans Avoir, who was lieutenant to Peter and led a separate army.

At a local level, the preaching of the First Crusade ignited the Rhineland massacres perpetrated against Jews, which some historians have deemed "the first Holocaust". At the end of 1095 and beginning of 1096, months before the departure of the official crusade in August, there were attacks on Jewish communities in France and Germany. In May 1096, Emicho of Flonheim (sometimes incorrectly known as Emicho of Leiningen) attacked the Jews at Speyer and Worms. Other unofficial crusaders from Swabia, led by Hartmann of Dillingen, along with French, English, Lotharingian and Flemish volunteers, led by Drogo of Nesle and William the Carpenter, as well as many locals, joined Emicho in the destruction of the Jewish community of Mainz at the end of May. In Mainz, one Jewish woman killed her children rather than see them killed; the chief rabbi, Kalonymus Ben Meshullam, committed suicide in anticipation of being killed.Emicho's company then went on to Cologne, and others continued on to Trier, Metz, and other cities. Peter the Hermit may have been involved in violence against the Jews, and an army led by a priest named Folkmar also attacked Jews further east in Bohemia.

The voyage towards Constantinople started peaceful but met with some conflicts in Hungary, Serbia, Nis. King Coloman the Learned, had to deal with the problems that the armies of the First Crusade caused during their march across Hungary towards the Holy Land in 1096. He defeated and massacred two crusader hordes to prevent their pillaging raids in Kingdom of Hungary. Emicho's army eventually continued into Hungary but was defeated by the army of Coloman. Emicho's followers dispersed; some eventually joined the main armies, although Emicho himself went home.

Walter Sans Avoir, a few thousand French crusaders left before Peter and reached Hungary on 8 May, passing through Hungary without incident and arriving at the river Sava at the border of the Byzantine territory at Belgrade. The Belgrade commander was taken by surprise, having no orders on what to do with them, and refused entry, forcing the crusaders to pillage the countryside for food. This resulted in skirmishes with the Belgrade garrison and, to make matters worse, sixteen of Walter's men had tried to rob a market in Zemun across the river in Hungary and were stripped of their armor and clothing, which was hung from the castle walls.

In Zemun, the crusaders became suspicious, seeing Walter's sixteen suits of armor hanging from the walls, and eventually a dispute over the price of a pair of shoes in the market led to a riot, which then turned into an all-out assault on the city by the crusaders, in which 4,000 Hungarians were killed. The crusaders then fled across the river Sava to Belgrade, but only after skirmishing with Belgrade troops. The residents of Belgrade fled, and the crusaders pillaged and burned the city.

Then they marched for seven days, arriving at Niš on 3 July. There, the commander of Niš promised to provide escort for Peter's army to Constantinople as well as food, if he would leave right away. Peter obliged, and the next morning he set out. However, a few Germans got into a dispute with some locals along the road and set fire to a mill, which escalated out of Peter's control until Niš sent out its entire garrison against the crusaders. The crusaders were completely routed, losing about 10,000 (a quarter of their number), the remainder regrouping further on at Bela Palanka. When they reached Sofia on 12 July they met their Byzantine escort, which brought them safely the rest of the way to Constantinople by 1 August.

They reached Constantinople by 1 August. Byzantine Emperor Alexius I Comnenus, not knowing what else to do with such an unusual and unexpected "army", quickly ferried all 30,000 across the Bosporus by 6 August. Alexius warned Peter not to engage the Turks, whom he believed to be superior to Peter's motley army, and to wait for the main body of crusaders, which was still on the way.

Peter was rejoined by the French under Walter Sans-Avoir and a number of bands of Italian crusaders who had arrived at the same time. Once in Asia Minor, they began to pillage towns and villages until they reached Nicomedia, where an argument broke out between the Germans and Italians on one side and the French on the other. The Germans and Italians split off and elected a new leader, an Italian named Rainald, while for the French, Geoffrey Burel took command. Peter had effectively lost control of the crusade.

Back at the main crusaders' camp, two Turkish spies had spread rumors that the Germans who had taken Xerigordos had also taken Nicaea, which caused excitement to get there as soon as possible to share in the looting. Three miles from the camp, where the road entered a narrow, wooded valley near the village of Dracon, the Turkish army was waiting. When approaching the valley, the crusaders marched noisily and were immediately subjected to a hail of arrows. Panic set in immediately and within minutes, the army was in full rout back to the camp. Most of the crusaders were slaughtered; however, women, children, and those who surrendered were spared. Eventually the Byzantines under Constantine Katakalon sailed over and raised the siege; these few thousand returned to Constantinople, the only survivors of the People's Crusade.

The four main crusader armies left Europe around the appointed time in August 1096. They took different paths to Constantinople and gathered outside its city walls between November 1096 and April 1097. The size of the entire crusader army is difficult to estimate. The princes arrived in Constantinople with little food and expected provisions and help from Alexios. Alexios was understandably suspicious after his experiences with the People's Crusade, and also because the knights included his old Norman enemy, Bohemond, who had invaded Byzantine territory on numerous occasions with his father, Robert Guiscard, and may have even attempted to organize an attack on Constantinople while encamped outside the city. The crusaders may have expected Alexios to become their leader, but he had no interest in joining them, and was mainly concerned with transporting them into Asia Minor as quickly as possible. In return for food and supplies, Alexios requested the leaders to swear fealty to him and promise to return to the Byzantine Empire any land recovered from the Turks. Before ensuring that the various armies were shuttled across the Bosporus, Alexios advised the leaders on how best to deal with the Seljuq armies that they would soon encounter.

The crusaders began to leave Constantinople at the end of April 1097. Godfrey of Bouillon was the first to arrive at Nicaea, with Bohemond of Taranto, Bohemond's nephew Tancred, Raymond IV of Toulouse, and Robert II of Flanders following him, along with Peter the Hermit and some of the survivors of the People's Crusade, and a small Byzantine force under Manuel Boutoumites. They arrived on 6 May, severely short of food, but Bohemond arranged for food to be brought by land and by sea. They put the city to siege beginning on 14 May, assigning their forces to different sections of the walls, which was well-defended with 200 towers. Bohemond camped on the north side of the city, Godfrey on the south, and Raymond and Adhemar of Le Puy on the eastern gate.

On 16 May the Turkish defenders sallied out to attack the crusaders, but the Turks were defeated in a skirmish with the loss of 200 men. The Turks sent messages to Kilij Arslan begging him to return, and when he realized the strength of the crusaders he quickly turned back. An advance party was defeated by troops under Raymond and Robert II of Flanders on 20 May, and on 21 May the crusader army defeated Kilij in a pitched battle which lasted long into the night. Losses were heavy on both sides, but in the end the sultan retreated despite the pleas of the Nicaean Turks. The rest of the crusaders arrived throughout the rest of May, with Robert Curthose and Stephen of Blois arriving at the beginning of June. Meanwhile, Raymond and Adhemar built a large siege engine, which was rolled up to the Gonatas Tower in order to engage the defenders on the walls while miners mined the tower from below. The tower was damaged but no further progress was made.

Byzantine emperor Alexios I chose not to accompany the crusaders, but marched out behind them and made his camp at nearby Pelecanum. From there, he sent boats, rolled over the land, to help the crusaders blockade Lake Ascanius, which had up to this point been used by the Turks to supply Nicaea with food. The boats arrived on 17 June, under the command of Manuel Boutoumites. The general Tatikios was also sent, with 2,000 foot soldiers. Alexios had instructed Boutoumites to secretly negotiate the surrender of the city without the crusaders' knowledge. Tatikios was instructed to join with the crusaders and make a direct assault on the walls, while Boutoumites would pretend to do the same to make it look as if the Byzantines had captured the city in battle. This was done, and on 19 June the Turks surrendered to Boutoumites.

When the crusaders discovered what Alexios had done, they were quite angry, as they had hoped to plunder the city for money and supplies. Boutoumites, however, was named dux of Nicaea and forbade the crusaders from entering in groups larger than 10 men at a time. Boutoumites also expelled the Turkish generals, whom he considered just as untrustworthy. Kilij Arslan's family went to Constantinople and were eventually released without ransom. Alexios gave the crusaders money, horses, and other gifts, but the crusaders were not pleased with this, believing they could have had even more if they had captured Nicaea themselves. Boutoumites would not permit them to leave until they had all sworn an oath of vassalage to Alexios, if they had not yet done so in Constantinople. As he had in Constantinople, Tancred at first refused, but he eventually gave in.

The crusaders left Nicaea on 26 June in two contingents: Bohemond, Tancred, Robert II of Flanders, and Tatikios in the vanguard, and Godfrey, Baldwin of Boulogne, Stephen, and Hugh of Vermandois in the rear. Tatikios was instructed to ensure the return of captured cities to the empire. Their spirits were high, and Stephen wrote to his wife Adela that they expected to be in Jerusalem in five weeks.

The crusaders had left Nicaea on 26 June 1097, with a deep distrust of the Byzantines, who had taken the city without their knowledge after the lengthy siege of Nicaea. In order to simplify the problem of supplies, the Crusader army had split into two groups; the weaker led by Bohemond of Taranto, his nephew Tancred, Robert Curthose, Robert of Flanders, and the Byzantine general Tatikios in the vanguard, and Godfrey of Bouillon, his brother Baldwin of Boulogne, Raymond IV of Toulouse, Stephen II of Blois, and Hugh of Vermandois in the rear.

On 29 June, they learnt that the Turks were planning an ambush near Dorylaeum (Bohemond noticed that his army was being shadowed by Turkish scouts). The Turkish force, consisting of Kilij Arslan and his ally Hasan of Cappadocia, along with help from the Danishmends, led by the Turkish prince Gazi Gümüshtigin. Contemporary figures place the number of Turks as between 25,000–30,000, with more recent estimates are between 6,000 and 8,000 men.

The Battle of Dorylaeum took place during the First Crusade on 1 July 1097, between the Seljuk Turks and the Crusaders, near the city of Dorylaeum in Anatolia. Despite the Turkish forces of Kilij Arslan almost destroying the Crusader contingent of Bohemond, other Crusaders arrived just in time for a very close victory.

The crusaders did indeed become rich, at least for a short time, after capturing Kilij Arslan's treasury. The Turks fled and Arslan turned to other concerns in his eastern territory.

After the Battle of Dorylaeum, the crusaders were allowed to march virtually unopposed through Anatolia on their way to Antioch. It took almost three months to cross Anatolia in the heat of the summer, and in October they began the siege of Antioch. The crusaders arrived outside the city on 21 October and began the siege. The garrison sortied unsuccessfully on 29 December. After stripping the surrounding area of food, the crusaders were forced to look farther afield for supplies, opening themselves to ambush.

While the main crusader army was marching across Asia Minor in 1097, Baldwin and the Norman Tancred launched a separate expedition against Cilicia. Tancred tried to capture Tarsus in September, but Baldwin forced him to leave it, which gave rise to an enduring conflict between them. Baldwin seized important fortresses in the lands to the west of the Euphrates with the assistance of local Armenians.

The Armenian lord of Edessa, Thoros, sent envoys—the Armenian bishop of Edessa and twelve leading citizens—to Baldwin in early 1098, seeking his assistance against the nearby Seljuqs rulers. Being the first town to convert to Christianity, Edessa had played an important role in Christian history. Baldwin left for Edessa in early February, but troops sent by Balduk, the emir of Samosata, or Bagrat prevented him from crossing the Euphrates. His second attempt was successful and he reached Edessa on 20 February. Baldwin did not want to serve Thoros as a mercenary. The Armenian townspeople feared that he was planning to leave the town, so they persuaded Thoros to adopt him. Strengthened by troops from Edessa, Baldwin raided Balduk's territory and placed a garrison in a small fortress near Samosata.

Unlike the majority of the Armenians, Thoros adhered to the Orthodox Church, which made him unpopular among his Monophysite subjects. Shortly after Baldwin's return from campaign, the local nobles started plotting against Thoros, possibly with Baldwin's consent (as is stated by contemporary chronicler Matthew of Edessa). A riot broke out in the town, forcing Thoros to take refuge in the citadel. Baldwin pledged to save his adoptive father, but when the rioters broke into the citadel on 9 March and murdered both Thoros and his wife, he did nothing to help them. On the following day, after the townspeople acknowledged Baldwin as their ruler (or doux), he assumed the title of Count of Edessa, and so established the first Crusader state.

To strengthen his rule, the widowed Baldwin married an Armenian ruler's daughter (who is now known as Arda). He supplied the main crusader army with food during the siege of Antioch. He defended Edessa against Kerbogha, the governor of Mosul, for three weeks, preventing him from reaching Antioch before the crusaders captured it.

Bohemond persuaded the other leaders that if Antioch fell he would keep it for himself and that an Armenian commander of a section of the cities walls had agreed to enable the crusaders to enter. Stephen of Blois had been his only competitor and while deserting his message to Alexius that the cause was lost persuaded the Emperor to halt his advance through Anatolia at Philomelium before returning to Constantinople. Alexius failure to reach the siege was used by Bohemond to rationalise his refusal to return the city to the Empire as promised. The Armenian, Firouz, helped Bohemond and a small party enter the city on the 2nd June and open a gate at which point horns were sounded, the city's Christian majority opened the other gates and the crusaders entered. In the sack they killed most of the Muslim inhabitants and many Christian Greeks, Syrians and Armenians in the confusion.

The besiegers have become the besieged. On 4 June the vanguard of Kerbogha's 40,000 strong army arrived surrounding the Franks. From 10 June for 4 days waves of Kerbogha's men assailed the city walls from dawn until dusk. Bohemond and Adhemar barred the city gates to prevent mass desertions and managed to hold out. Kerbogha then changed tactics to trying to starve the crusaders out.

Morale inside the city was low and defeat looked imminent but a peasant visionary called Peter Bartholomew claimed the apostle St Andrew came to him to show the location of the Holy Lance that had pierced Christ on the cross. This supposedly encouraged the crusaders but the accounts are misleading as it was two weeks before the final battle for the city. On 24 June the Franks sought terms for surrender that were refused. On 28 June 1098 at dawn the Franks marched out of the city in four battle groups to engage the enemy. Kerbogha allowed them to deploy with the aim of destroying them in the open. However the discipline of the Muslim army did not hold and a disorderly attack was launched. Unable to overrun a bedraggled force they outnumbered two to one Muslims attacking the Bridge Gate fled through the advancing main body of the Muslim army. With very few casualties the Muslim army broke and fled the battle.

The crusaders reached Jerusalem, which had been recaptured from the Seljuqs by the Fatimids only the Year before, on 7 June. Many Crusaders wept upon seeing the city they had journeyed so long to reach. The Fatimid governor Iftikhar al-Dawla prepared the city for the siege as he heard about the arrival of the Crusaders. He prepared an elite troop of 400 Egyptian cavalry men and had expelled all the eastern Christians from the city due to the fear of betrayal from them (in the siege of Antioch an Armenian man named Firoz helped crusaders enter the city by opening the gates). To make the situation worse for the crusaders, ad-Daula poisoned or buried all the water wells, and cut down all trees outside Jerusalem. On 7 June 1099, the crusaders reached outside the fortifications of Jerusalem, which had been recaptured from the Seljuqs by the Fatimids only the Year before. The city was guarded by a defense wall 4 km long, 3 meters thick and 15 meters high, there were five major gates each guarded by a pair of towers. The Crusaders divided themselves into two large groups- Godfrey of Bouillon, Robert of Flanders and Tancred planned to besiege from north while, Raymond of Toulouse positioned his forces on the south.

A small fleet of Genoese and English ships arrives at the port of Jaffa bringing essential supplies for siege weapons to the First Crusaders at Jerusalem. The Genoese sailors had brought all the necessary equipment with them for the construction of the siege equipments.

Robert of Normandy and Robert of Flanders procured timber from the nearby forests. Under the command of Guglielmo Embriaco and Gaston of Béarn, the crusaders began the construction of their siege weapons. They constructed the finest siege equipment of the 11th century in almost 3 weeks. This included: 2 massive wheel-mounted siege towers, a battering ram with an iron-clad head, numerous scaling ladders and a series of portable wattle screens. On the other hand the Fatimids kept an eye on the preparation by the Franks and they set up their mangonels on the wall in the firing range once an assault began. The preparation by the crusaders was complete.

On 14 July 1099, the crusaders launched their attack, Godfrey and his allies were positioned towards the Northern wall of Jerusalem, their priority was to break through the outer curtain of the walls of Jerusalem. By the end of the day they penetrated the first line of defense. On the South Raymond's (of Toulouse) forces were met with ferocious resistance by the Fatimids. On 15 of July the assault recommenced in the Northern front, Godfrey and his allies gained success and the crusader Ludolf of Tournai was the first to mount the wall. The Franks quickly gained the foothold in the wall, and as the city's defenses collapsed, waves of panic shook the Fatimids.

The crusaders made their way into the city through the tower of David and history witnessed one of the most bloody encounters. The crusaders massacred every inhabitant of the city (Jerusalem), Muslims and Jews alike.

On 22 July, a council was held in the Church of the Holy Sepulchre to establish governance for Jerusalem. Godfrey of Bouillon (who played the most fundamental role in the city's conquest) was made Advocatus Sancti Sepulchri ("advocate" or "defender of the Holy Sepulchre").

The Battle of Ascalon took place on 12 August 1099 shortly after the capture of Jerusalem, and is often considered the last action of the First Crusade. The crusader army led by Godfrey of Bouillon defeated and drove off a Fatimid army, securing the safety of Jerusalem.

The majority of crusaders now considered their pilgrimage complete and returned home. Only 300 knights and 2,000 infantry remained to defend Palestine. Relations between the newly created Crusader states of the County of Edessa and Principality of Antioch were variable. The Franks became fully engaged in Near East politics with the result that Muslims and Christians often fought each other. Antioch's territorial expansion ended in 1119 with a major defeat to the Turks at the Battle of Ager Sanguinis, the Field of Blood.