Kazimierz I was the son of Mieszko II and Ryksa and was born in the year 1016. His father entitled him to a monastery to be raised (most likely a German one), and he fulfilled his vows as a priest under the name of Karol. After Mieszko's death, his wife Ryksa wanted Kazimierz to stay in Germany, but Kazimierz, obtaining Papal permission to be released from his religious vows, returned to Poland.

In the country he found the revolting of the people along with the effort to erradicate Christian practices and a push to go back to paganism. Gniezno and Poznań were devastated under the Czech priest Brzetysław. Kazimierz was fortunate to obtain political and military help from the German emperor Konrad II who supported his suppression of the rising strength of the Czechs.

The task before Kazimierz was not an easy one. First he suppressed and quieted the separatist tribal factions in the country, and he also unified Poland with Mazovia and in 1050 he regained Silesia from the Czechs. He strove to regain only these lands and tried to maintain peace and ensure the strength of his rule. He did not try to gain complete sovereignty from Germany, but still ruled Poland with complete power. He married a Russian princess who was the sister of the Ukranian ruler Jarosław the Wise. The marriage ensured peace on the eastern border of the realm and gave Kazimierz some military help against the Mazovian factions.

Chroniclers gave Kazimierz the name of Odnowiciel (The Renewer) because of his ability to reorganize the country both politically and religiously. He renewed the military power of the land and gave all Knights land in exchange for military service to the Crown. He built monasteries, churches, and enabled the return of the archbishop to Gniezno and Poznań. He also helped found a new Archbishopric in Krak�w.

Kazimierz I died in the year 1058 and one of his daughters, Świętosława, later became the wife of the Czech king Wratysław II.