Pope Callixtus II or Callistus II, born Guy of Burgundy, was head of the Catholic Church and ruler of the Papal States from 1 February 1119 to his death in 1124. His pontificate was shaped by the Investiture Controversy, which he was able to settle through the Concordat of Worms in 1122.

The Investiture Controversy or Investiture Contest was a conflict between the Church and the state in medieval Europe over the ability to choose and install bishops (investiture) and abbots of monasteries and the pope himself. A series of popes in the 11th and 12th centuries undercut the power of the Holy Roman Emperor and other European monarchies, and the controversy led to nearly 50 years of conflict.