Darío del Niño Jesús Castrillón Hoyos was a Colombian cardinal of the Catholic Church. He was Prefect of the Congregation for the Clergy from 1996 to 2006 and President of the Pontifical Commission Ecclesia Dei from 2000 until his retirement in 2009. He was made a cardinal in 1998.

A cardinal is a senior member of the clergy of the Catholic Church. Cardinals are created by the pope and typically hold the title for life. Collectively, they constitute the College of Cardinals. The most solemn responsibility of the cardinals is to elect a new pope in a conclave, almost always from among themselves, when the Holy See is vacant. During the period between a pope's death or resignation and the election of his successor, the day-to-day governance of the Holy See is in the hands of the College of Cardinals. The right to participate in a conclave is limited to cardinals who have not reached the age of 80 years by the day the vacancy occurs. In addition, cardinals collectively participate in papal consistories, in which matters of importance to the Church are considered and new cardinals may be created. Cardinals of working age are also appointed to roles overseeing dicasteries of the Roman Curia, the central administration of the Catholic Church.