Simony is the ecclesiastical crime and personal sin of paying for offices or positions in the hierarchy of a church, named after Simon Magus, who appears in the Acts of the Apostles 8:18-24. Simon Magus offers the disciples of Christ payment for the power to perform miracles.

The intertwining of temporal with spiritual authority in the Middle Ages caused endless problems with simony and accusations of simony. Secular rulers wanted to employ the educated and centrally organized clergy in their administrations, and often treated their spiritual positions as adjuncts to the secular administrative roles.

Canon law also outlawed as simony some acts that did not involve the sale of offices, but the sale of spiritual authority: the sale of tithes, the taking of a fee for confession, absolution, marriage or burial, and the concealment of one in mortal sin or the reconcilement of an impenitent for the sake of gain. Just what was or was not simony was strenuously litigated: as one commentator notes, the widespread practice of simony is best evidenced by the number of reported ecclesiastical decisions as to what is or is not simony.

Simony did serious harm to the moral standing of the Catholic Church. Dante Alighieri condemns simonists to the eighth circle of hell in his Inferno, where he encounters Pope Nicholas III buried upside down, the soles of his feet burning with an undefined oily substance. Nicholas goes on to predict the damnation of both Pope Boniface VIII, the Pope in office at the time the Divine Comedy is set, and Pope Clement V, his successor, for that sin. Less devout writers, such as Machiavelli and Erasmus, condemned the practice centuries later, while Blaise Pascal attacked the casuistic defenses offered by those accused of simony in his "Provincial Letters".

The Church of England also struggled with the practice after its separation from the Catholic Church. While English law recognized simony as an offense, it treated it as merely an ecclesiastical matter, rather than a crime, for which the punishment was forfeiture of the office or any advantage from the offense and severance of any patronage relationship with the person who bestowed the office.