A Catholic priest celebrating mass
Catholic priests are ordained by bishops through the sacrament of holy orders. The Catholic Church claims that Catholic bishops were ordained in an unbroken line of apostolic succession back to the Twelve Apostles depicted in the Catholic Bible. The ceremony of Eucharist, which Catholics believe can only be performed 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 a bishop and a presbyter as "priests".
Various churches which split off from the Catholic Church make the same claim of apostolic succession, including the Church of the East (split 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 priests. This was not universally accepted, contributing to the schism of various Protestant churches. The doctrine is interpreted in various ways by different protestant denominations, with some dropping apostolic succession and holy orders as a sacrament, and different requirements for who can perform the Eucharist ceremony. Through the principle of church economy, the Catholic Church recognizes as valid the ordination of priests in denominations with unbroken apostolic succession, such as in the Eastern Orthodox Church, Polish National Catholic Church, Oriental Orthodox Church, the Assyrian Church of the East, Church of Sweden, and Evangelical Lutheran Church of Finland, but not other Lutheran churches. 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.
In 1965, the Second Vatican Council released Presbyterorum Ordinis on the ministry and life of priests, and Optatam Totius on the training of priests.
Since 1970, the number of Catholic priests in the world has decreased by only about 5,000, to 414,313 priests as of 2012.but the worldwide Catholic population has nearly doubled, growing from 653.6 million in 1970 to 1.229 billion in 2012. This has resulted in a worldwide shortage of Catholic priests. In 2014, 49,153 Catholic parishes had no resident priest pastor. The number of priests is increasing in Africa and Asia, but not keeping pace with growth in Catholic populations there. The number of priests is falling in Europe and the Americas faster than the number of local Catholics is declining. This has resulted in some African and Asian priests being recruited to European and American churches, reversing the historical practice of Catholic missionaries being sent from Western countries to the rest of the world.
In the 1990s and 2000s, the cases of sexual abuse by Catholic priests gained worldwide attention, with thousands of accused priests and tens of thousands of alleged victims. The church estimated that over the 50 years ending in 2009, between 1.5% and 5% of Catholic priests had a sexual encounter with a minor,  and Dr. Thomas Plante estimated a figure of 4%. Public anger was fueled by the revelation that many accused priests were transferred to another parish rather than being removed from ministry or reported to police. The scandal caused some Catholics to leave the church, made recruitment of new priests more difficult, and resulted in billions of dollars in lawsuit settlements and bankruptcies that increased financial pressure to close parishes with declining membership. In February 2019, clerical abuse of nuns, including sexual slavery, has been acknowledged by the Pope.