At its greatest extent, in the mid-11th century, it stretched from the Baltic Sea in the north to the Black Sea in the south and from the headwaters of the Vistula in the west to the Taman Peninsula in the east, uniting the majority of East Slavic tribes.
According to Russian historiography, the first ruler to start uniting East Slavic lands into what has become known as Kievan Rus' was Prince Oleg (882–912). He extended his control from Novgorod south along the Dnieper river valley to protect trade from Khazar incursions from the east, and he moved his capital to the more strategic Kiev. Sviatoslav I (died 972) achieved the first major expansion of Kievan Rus' territorial control, fighting a war of conquest against the Khazars. Vladimir the Great (980–1015) introduced Christianity with his own baptism and, by decree, extended it to all inhabitants of Kiev and beyond. Kievan Rus' reached its greatest extent under Yaroslav the Wise (1019–1054); his sons assembled and issued its first written legal code, the Rus' Justice, shortly after his death.
The state declined beginning in the late 11th century and during the 12th century, disintegrating into various rival regional powers. It was further weakened by economic factors, such as the collapse of Rus' commercial ties to Byzantium due to the decline of Constantinople and the accompanying diminution of trade routes through its territory. The state finally fell to the Mongol invasion of the 1240s.
During its existence, Kievan Rus' was known as the "land of the Rus'" (Old East Slavic: Рѹ́сьскаѧ землѧ, from the ethnonym Рѹ́сь; Greek: Ῥῶς; Arabic: الروس al-Rūs), in Greek as Ῥωσία, in Old French as Russie, Rossie, in Latin as Russia (with local German spelling variants Ruscia and Ruzzia), and from the 12th century also Ruthenia. Various etymologies have been proposed, including Ruotsi, the Finnish designation for Sweden, and Ros, a tribe from the middle Dnieper valley region.
The term Kievan Rus' (Ки́евская Русь Kievskaya Rus’) was coined in the 19th century in Russian historiography to refer to the period when the centre was in Kiev. In English, the term was introduced in the early 20th century, when it was found in the 1913 English translation of Vasily Klyuchevsky's A History of Russia, to distinguish the early polity from successor states, which were also named Rus. Later, the Russian term was rendered into Belarusian and Ukrainian as Кіеўская Русь (Kijeŭskaja Rus’) and Ки́ївська Русь (Kyivs'ka Rus’), respectively.
Prior to the emergence of Kievan Rus' in the 9th century AD, the lands between the Baltic Sea and Black Sea were primarily populated by eastern Slavic tribes. In the northern region around Novgorod were the Ilmen Slavs and neighboring Krivichi, who occupied territories surrounding the headwaters of the West Dvina, Dnieper, and Volga Rivers. To their north, in the Ladoga and Karelia regions, were the Finnic Chud tribe. In the south, in the area around Kiev, were the Poliane, a group of Slavicized tribes with Iranian origins, the Drevliane to the west of the Dnieper, and the Severiane to the east. To their north and east were the Vyatichi, and to their south was forested land settled by Slav farmers, giving way to steppelands populated by nomadic herdsmen.
Controversy persists over whether the Rus' were Varangians (Vikings) or Slavs. This uncertainty is due largely to a paucity of contemporary sources. Attempts to address this question instead rely on archaeological evidence, the accounts of foreign observers, and legends and literature from centuries later. To some extent the controversy is related to the foundation myths of modern states in the region. According to the "Normanist" view, the Rus' were Scandinavians, while Russian and Ukrainian nationalist historians generally argue that the Rus' were themselves Slavs. Normanist theories focus on the earliest written source for the East Slavs, the Primary Chronicle, although even this account was not produced until the 12th century. Nationalist accounts have suggested that the Rus' were present before the arrival of the Varangians, noting that only a handful of Scandinavian words can be found in modern Russian and that Scandinavian names in the early chronicles were soon replaced by Slavic names. Nevertheless, archaeological evidence from the area suggests that a Scandinavian population was present during the 10th century at the latest. On balance, it seems likely that the Rus' proper were a small minority of Scandinavians who formed an elite ruling class, while the great majority of their subjects were Slavs. Considering the linguistic arguments mounted by nationalist scholars, if the proto-Rus' were Scandinavians, they must have quickly become nativized, adopting Slavic languages and other cultural practices.