The Investiture Controversy 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.

It began as a power struggle between Pope Gregory VII and Henry IV (then King, later Holy Roman Emperor) in 1076. Gregory VII even enlisted Normans under Robert Guiscard(the Norman ruler of Sicily, Apulia, and Calabria) in the struggle. 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.

As an aftermath of this struggle, the papacy grew stronger, and the laity became engaged in religious affairs, increasing its piety and setting the stage for the Crusades and the great religious vitality of the 12th century. Though the Holy Roman Emperor retained some power over imperial churches, his power was damaged irreparably because he lost the religious authority that previously belonged to the office of the king.