Opus Dei (Latin for 'Work of God') is a personal prelature of the Catholic Church founded in Spain on 2 October 1928 by Josemaría Escrivá. The prelature states that it helps lay and clerical members pursue holiness through everyday occupations and social responsibilities. The Holy See granted final approval in 1950 under Pope Pius XII. In 1982 Pope John Paul II affirmed its status as a personal prelature through the apostolic constitution Ut sit. Catholic Church leaders have voiced institutional support while the organization remains controversial.

Laypeople constitute most members of Opus Dei, and the rest are secular priests governed by a prelate elected by specific members and appointed by the Pope. Because Opus Dei is Latin for "Work of God", members and supporters often refer to the prelature as "the Work". Beyond personal charity and social services, the prelature organizes training in Catholic spirituality applied to daily life. Opus Dei members are located in more than 90 countries. About 70% of members live in their own homes and maintain secular careers while taking part in prelature activities, while the remaining are celibate, of whom the majority live in Opus Dei centers.

Opus Dei was founded by Josemaría Escrivá on 2 October 1928 in Madrid, Spain. According to Escrivá, on that day he experienced a vision in which he "saw Opus Dei". He gave the organization the name "Opus Dei", which in Latin means "Work of God", in order to underscore the belief that the organization was not his (Escrivá's) work, but was rather God's work. Throughout his life, Escrivá held that the founding of Opus Dei had a supernatural character. Escrivá summarized Opus Dei's mission as a way of helping ordinary Christians "to understand that their life ... is a way of holiness and evangelization ... And to those who grasp this ideal of holiness, the Work offers the spiritual assistance and training they need to put it into practice."

Initially, Opus Dei was open only to men, but in 1930, Escrivá started to admit women, based on what he believed to be a communication from God. Persecuted during the Spanish Civil War and narrowly escaping death several times, in 1939 Escrivá was able to return to Madrid after three years of hiding. In 1939, Escrivá published The Way, a collection of 999 maxims concerning spirituality for people involved in secular affairs.

Opus Dei rapidly grew during the years of the Francoist dictatorship, spreading first throughout Spain, and after 1945, expanding internationally. Escrivá had to overcome many obstacles. He later recounted that it was in Spain where Opus Dei found "the greatest difficulties" because of "enemies of personal freedom", and traditionalists who he felt misunderstood Opus Dei's ideas.

In 1947, a year after Escrivá moved the organization's headquarters to Rome, Opus Dei received a decree of praise and approval from Pope Pius XII, making it an institute of "pontifical right", i.e. under the direct governance of the Pope. In 1950, Pius XII granted definitive approval to Opus Dei, thereby allowing married people to join the organization, and secular clergy to be admitted to the Priestly Society of the Holy Cross. Several Opus Dei members such as Alberto Ullastres were ministers under the dictator Francisco Franco in Spain (see Opus Dei and politics).

In 1975, Escrivá died and was succeeded by Álvaro del Portillo. In 1982, Opus Dei was made into a personal prelature. This means that Opus Dei is part of the Catholic Church, and the spirituality the members falls under the direct jurisdiction of the prelate of Opus Dei wherever they are. As to "what the law lays down for all the ordinary faithful", the lay members of Opus Dei, being no different from other Catholics, "continue to be ... under the jurisdiction of the diocesan bishop", in the words of John Paul II's Apostolic Constitution, Ut Sit.

One-third of the world's bishops sent letters petitioning for the canonization of Escrivá. Escrivá was beatified in 1992 in the midst of controversy prompted by questions about his suitability for sainthood. In 2002, approximately 300,000 people gathered in St. Peter's Square on the day Pope John Paul II canonized him. There are other members whose process of beatification has been opened: Ernesto Cofiño, a father of five children and a pioneer in pediatric research in Guatemala; Montserrat Grases, a teenage Catalan student who died of cancer; Toni Zweifel, a Swiss engineer; Tomás Alvira and wife, Paquita Domínguez, a Spanish married couple; Isidoro Zorzano Ledesma, an Argentinian engineer; Dora del Hoyo, a domestic worker; Fr. José María Hernández Garnica; and Father José Luis Múzquiz de Miguel, a Spanish priest who began Opus Dei in the United States.