The priesthood is the office of the ministers of religion, who have been commissioned ("ordained") with the holy orders of the Catholic Church. Technically, bishops are a priestly order as well; however, in common English usage priest refers only to presbyters and pastors (parish priests). The church's doctrine also sometimes refers to all baptised members (inclusive of the laity) as the "common priesthood", which can be confused with the ministerial priesthood of the ordained clergy.

The church has different rules for priests in the Latin Church–the largest Catholic particular church–and in the 23 Eastern Catholic Churches. Notably, priests in the Latin Church must take a vow of celibacy, whereas most Eastern Catholic Churches permit married men to be ordained. Deacons are male and usually belong to the diocesan clergy, but, unlike almost all Latin Church (Western Catholic) priests and all bishops from Eastern or Western Catholicism, they may marry as laymen before their ordination as clergy. The priesthood is a vocation for men; a similar but distinct vocation for women is the sisterhood. The Catholic Church teaches that when a man participates in priesthood after the Sacrament of Holy Orders, he acts in persona Christi Capitis, representing the person of Christ.

Unlike usage in English, "the Latin words sacerdos and sacerdotium are used to refer in general to the ministerial priesthood shared by bishops and presbyters. The words presbyter, presbyterium and presbyteratus refer to priests in the English use of the word or presbyters." According to the Annuario Pontificio 2016, as of December 31, 2014, there were 415,792 Catholic priests worldwide, including both diocesan priests and priests in the religious orders. A priest of the regular clergy is commonly addressed with the title "Father" (contracted to Fr, in the Catholic and some other Christian churches). An example of how to title a catholic father is with their last or first name conjoined with father. Ex. Father Fleming.

Catholics living a consecrated life or monasticism include both the ordained and unordained. Institutes of consecrated life, or monks, can be deacons, priests, bishops, or non-ordained members of a religious order. The non-ordained in these orders are not to be considered laypersons in a strict sense—they take certain vows and are not free to marry once they have made solemn profession of vows. All female religious are non-ordained; they may be sisters living to some degree of activity in a communal state, or nuns living in cloister or some other type of isolation. The male members of religious orders, whether living in monastic communities or cloistered in isolation, and who are ordained priests or deacons constitute what is called the religious or regular clergy, distinct from the diocesan or secular clergy. Those ordained priests or deacons who are not members of some sort of religious order (secular priests) most often serve as clergy to a specific church or in an office of a specific diocese or in Rome.

Catholic priests are ordained by bishops through the sacrament of holy orders. Catholic bishops are ordained in an unbroken line of apostolic succession back to the Twelve Apostles depicted in the Catholic Bible. The ceremony of Eucharist, which can only be confected by priests, in particular derives from the story of the Last Supper, when Jesus Christ distributed bread and wine in the presence of the Twelve Apostles, in some versions of the Gospel of Luke commanding them to "do this in memory of me". (Some Protestant critics have challenged the historical accuracy of the claim of unbroken succession.)

Catholic tradition says the apostles in turn selected other men to succeed them as the bishops (episkopoi, Greek for "overseers") of the Christian communities, with whom were associated presbyters (presbyteroi, Greek for "elders") and deacons (diakonoi, Greek for "servants"). As communities multiplied and grew in size, the bishops appointed more and more presbyters to preside at the Eucharist in place of the bishop in the multiple communities in each region. The diaconate evolved as the liturgical assistants of the bishop and his delegate for the administration of church funds and programmes for the poor. Today, the rank of "presbyter" is typically what one thinks of as a priest, although church catechism considers both bishops and presbyters as "priests".

The Pentarchic Church of the three first Holy Synods share the tradition of the sacrament of ordination by which the grace of apostolic succession is secured; this includes the Church of the East (split from the Catholic Church in 424), the Oriental Orthodoxy (split in 451) and the Eastern Orthodox Church (split with the East–West Schism of 1054). During the Protestant Reformation, Martin Luther and William Tyndale advocated the priesthood of all believers, the idea that all baptized Christians are equally part of the sacred priesthood and that ministerial priesthood has no real authority beyond that of the congregation. This was a complex and controversial matter, contributing to further schisms within the Reformation movement of the Church. The Lutheran-Evangelical Swedish Church maintains the sacrament of ordination and the doctrine of Apostolic Succession, as does the Anglican Church. The doctrine is interpreted in various ways by different Protestant denominations, with some dropping apostolic succession and holy orders as a sacrament, as per example the Church of Norway and Denmark who keep their respective Monarchs as Pontiffs, sovereign heads of the church-hierarchy. There are different requirements for the performance of the Eucharistic ceremony to be valid among different kinds of Christian denominations, in regard of who are to oversee the sanctity of the Eucharist and stand as guarantor that the Holy Service is properly performed. There are significant differences particularly concerning the strictness/liberality of who is welcome to receive the sacraments.

Through the principle of church economy, the Catholic Church Norms at the same time recognizes as valid the Holy Service of denominations practicing the Nicene Creed (Symbolum Nicaenum), and deem illicit and therefore find the ordination of priests "objectively sacrilegious" in denominations separated from the one, holy, apostolic and catholic (i.e. universal) Church, which holds an unbroken apostolic succession. Alongside the Eastern Catholic Churches, it shares and defines the so-called First seven ecumenical councils. The Eastern Catholic Churches or Oriental Catholic Churches, also called the Eastern-rite Catholic Churches, Eastern Rite Catholicism, or simply the Eastern Churches are 23 Eastern Christian sui iuris (autonomous) particular churches of the Catholic Church, in full communion with the Bishop of Rome. Although they are distinct theologically, liturgically, and historically from the Latin Church, they are all in full communion with it and with each other. The Coptic Catholic Church among these 23 is not identical with the Coptic Orthodox Patriarchate of Alexandria. The Old Catholic church broke communion with the Roman Catholic Church in the 1870s and are thus schismatic. Furthermore, in 2008, the Old Catholics of the Union of Scranton broke away from the Union of Utrecht after the Union of Utrecht began ordaining women and blessing same-sex unions. Since then, the Union of Scranton has expanded to include the Nordic Catholic Church (NCC) which separated from the Church of Norway under sovereignty of the King of Norway, in opposition to similar practices, and has developed a more Catholic theology. The Nordic Catholic Church includes the Christ-Catholic Church of Germany as a daughter-church, which traces its history through the Old Catholics of the Union of Utrecht and the Polish-Catholic Church of the Republic of Poland. The Holy Apostolic Catholic Assyrian Congregation of the East holds apostolic succession, but is not in communion with either the Oriental Orthodox Congregation in full communion with the Roman or the Eastern Orthodox Church. In contrast to the Evangelical-Lutheran religion of Denmark and Norway, the Church of Sweden practices Apostolic Succession, and holds Ordination as a sacrament. Recognition of the ordination of Anglican Church priests was denied in 1896 by Pope Leo XIII through the papal bull Apostolicae curae over a dispute in the wording of the Anglican ceremony starting in the 1500s.