During the Early Middle Ages, the Karelians developed as an ethnic group in the region of present-day eastern Finland and northwestern Russia, primarily through the mixing of settlers from western Finland with the local population. Archaeological evidence suggests the highest concentration of Karelian settlements was along the western shore of Lake Ladoga and the Karelian Isthmus, with permanent habitation dating back around 600 CE to 800 CE. The Karelian population grew rapidly from 800 CE to 1050 CE, as evidenced by a significant increase in archaeological finds.
The first written mentions of Karelia and the Karelians appear in Scandinavian sources. Norse sagas from as early as the 7th century CE refer to Karelia by names such as Karjalabotn and Kirjaland. One key historical event involving the Karelians was their raid on Sigtuna, Sweden, in 1187 CE, an act that reportedly contributed to the founding of Stockholm.
In Russian chronicles, Karelians were first mentioned in 1143 CE, with records of Karelian raids on Tavastia (Häme). Over time, Karelia's relationship with Novgorod changed from alliance to dominance by Novgorod, particularly in the 13th century CE, with Karelians becoming increasingly involved in Novgorodian military and political affairs.