The Investiture Controversy was the most significant conflict between church and state in medieval Europe, lasting from 1076 to 1122.

The conflict centered on the practice of 'lay investiture,' where a secular ruler (like an emperor or king) invested a new bishop or abbot with the symbols of his spiritual office.

The Church's Gregorian Reform movement sought to end this practice to ensure church officials were loyal to the Pope, not to secular lords.

The main antagonists in the conflict were Pope Gregory VII and the Holy Roman Emperor Henry IV.

In 1075, Pope Gregory VII formally condemned lay investiture and excommunicated Henry IV for defying him.

Excommunication was a powerful political weapon, as it released a king's vassals from their oaths of loyalty.

In a famous act of public penance, Henry IV journeyed to the castle of Canossa in 1077 to beg the pope for forgiveness.

Despite the reconciliation at Canossa, the conflict between the papacy and secular rulers continued for decades.

The controversy was finally settled by a compromise known as the Concordat of Worms in 1122.

The Concordat of Worms established that bishops would be elected by the Church, but the emperor would have a say over the temporal (land-holding) aspects of the appointment, separating spiritual from secular authority.