Casimir I the Restorer, a member of the Piast dynasty, was the duke of Poland from 1040 until his death. Casimir was the son of Mieszko II Lambert and Richeza of Lotharingia. He is known as the Restorer because he managed to reunite parts of the Kingdom of Poland after a period of turmoil. He reincorporated Masovia, and conquered Silesia and Pomerania. However, he failed to crown himself King of Poland, mainly because of internal and external threats to his rule.

The House of Piast was the first historical ruling dynasty of Poland. The first documented Polish monarch was Duke Mieszko I. The Piasts' royal rule in Poland ended in 1370 with the death of King Casimir III the Great.