Yaroslav I, son of
Vladimir I, became Grand Prince of Kievan
Russia in
1019 after defeating his older brother
Sviatopolk I who had taken the throne on their father's death. With the support of Scandinavians and the Russians of
Novgorod, Yaroslav was able to defeat Sviatopolk, who had
Polish backing.
Yaroslav's reign is often considered the height of Kievan Russia's power, and he himself was nicknamed "Yaroslav the Wise." It was not a peaceful reign; he had to subdue a rebellion by his nephew Bryacheslav, and later to let his own brother Mstislav rule the eastern half of Russian territory (until Mstislav died in 1036 and Yaroslav became ruler again of all Russian lands). The two brothers were able to work together, though, to recapture lands that had been taken by Poland in return for having supported Sviatopolk. He was able to permanently defeat the Pecheneg tribes who had many times attacked Russia. He sent one of his sons to attack Constantinople, but after several years of fighting the Russians were defeated there and gave up (for a while) on controlling the southern end of the Black Sea.
Yaroslav encouraged building, fortifying the city of Kiev and building its great St. Sophia Cathedral, among other churches and monasteries. Kiev became a great religious center during his reign, though previously cities farther south and west had been the centers for Russian Orthodox Christians. He married a Swedish princess, and married his sisters and daughters to other European kings, including those as far away as France. He is considered the main person responsible for the first known Russian code of laws, The Russian Justice. And he was a patron of artists, architects, schools, and libraries.
In 1054, Yaroslav died, and his eldest living son Izyaslav I succeeded him as Grand Prince, but Yaroslav had left instructions as to which cities his other sons were to be the Prince of, and encouragement for all his sons to live in peace with each other. He seemed to have the idea that when his oldest son died, the next-oldest would become Grand Prince and that all his sons would change cities, moving up a slot in a progression toward the highest position of power. Unfortunately, this didn't really work out in real life.