The author focuses on the history and on the modus operandi of one of the most important ecclesiastic institutions, i.e. the Apostolic Penitentiary (Sacra Poenitentiaria Apostolica). The Apostolic Penitentiary is one of the three main tribunals of the Roman Curia, together with the Roman Rota (Sacra Rota Romana) and the Apostolic Signature (Signatura Apostolica). Although the penitentiary is referred to as the supreme court dealing with the matters of conscience, never did it administer justice. Therefore, the author believes that it should be addressed as the main tribunal of mercy in the Church. Theoretically, all worshippers, clergymen or laymen, were entitled to turn to the Penitentiary asking for special dispensation from observing the canonical law. They only had to go to Rome personally or to send an authorized person instead and to present an adequate supplication addressed to the pope. The Penitentiary offered dispensations, absolutions, licences and exemptions from ecclesiastic punishment reserved to the pope himself, including the exemption from excommunication. The author referres to a number of examples from the historical sources in order to illustrate the ways in which the Penitentiary was used in the late Middle Ages by the sinners mainly originating from the ecclesiastical environments to purify them from a real or an alleged blame. The dishonest clergymen also mediated the so-called "difficult cases" in the Curia. Some of them even resorted to falsifying the documents issued by the Penitentiary.