The confessors of the Spanish kings are reputed to have a strong political influence because of their personal relationship with the monarchs, but the realities of their ministry are still largely ignored.This article studies the case of the dominican Luis Aliaga, confessor of Philip III from 1608 to 1621.Protected by the cardinal Javierre,whom he succeeded to the office of king’s confessor,L.Aliaga offers an interesting example of the diversity of the confessor’s activities. Besides the frequent meetings with the king to hear his confession,he acted as the protector of the king’s conscience, through a routine task of adviser concerning the affairs for which he was consulted by the organs of the government,and the participation to various councils and juntas, of finance among others. The insertion in the organs of the government appears to be a particularity of the confessors of the Spanish kings, whereas it was rather infrequent to hold concurrently the most important charges (member of the State council, General Inquisitor) as did L. Aliaga at he end of his career. The king’s confessor was also a real minister of religious affairs. His activity covers especially three fields: the provision of ecclesiastical benefits depending from the royal patronage, the internal affairs of the Spanish Church, and the relations with Rome. Through this work, the king’s confessor acted as a mediator in the conflicts concerning the clergy of Spain, and gave an important contribution to the diplomatic relations with the papacy. Nevertheless, the confessor of Philip III was frequently part of a collective process of advice, for instance in the administration of benefits or in the resolution of the cases of conscience related to the most important state affairs, and he was not powerful enough to modify the political orientation of the Monarchy, even in religious matters.