The Investiture Controversy, also called Investiture Contest (German: Investiturstreit; pronounced [nvstitutat] (listen)), 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 civil war in Germany.

It began as a power struggle between Pope Gregory VII and Henry IV (then King, later Holy Roman Emperor) in 1076. The conflict ended in 1122, when Pope Callixtus II and Emperor Henry V agreed on the Concordat of Worms. The agreement required bishops to swear an oath of fealty to the secular monarch, who held authority "by the lance" but left selection to the church. It affirmed the right of the church to invest bishops with sacred authority, symbolized by a ring and staff. In Germany (but not Italy and Burgundy), the Emperor also retained the right to preside over elections of abbots and bishops by church authorities, and to arbitrate disputes. Holy Roman Emperors renounced the right to choose the pope.

In the meantime, there was also a brief but significant investiture struggle between Pope Paschal II and King Henry I of England from 1103 to 1107. The earlier resolution to that conflict, the Concordat of London, was very similar to the Concordat of Worms.

Pope Callixtus II or Callistus II (c. 1065 – 13 December 1124), 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.

As son of Count William I of Burgundy, Guy was a member of and connected to the highest nobility in Europe. He became archbishop of Vienne and served as papal legate to France. He attended the Lateran Synod of 1112. He was elected pope at Cluny in 1119. The following year, prompted by attacks on Jews, he issued the bull Sicut Judaeis which forbade Christians, on pain of excommunication, from forcing Jews to convert, from harming them, from taking their property, from disturbing the celebration of their festivals, and from interfering with their cemeteries. In March 1123, Calixtus II convened the First Lateran Council which passed several disciplinary decrees, such as those against simony and concubinage among the clergy, and violators of the Truce of God.