The Society of Jesus (Latin: Societas Iesu, S.J. and S.I. or SJ, SI) is a religious order of men called Jesuits, who follow the teachings of the Catholic Church. Jesuit priests and brothers — also sometimes known colloquially as "God's marines" — are engaged in evangelization and apostolic ministry in 112 nations on six continents reflecting the Formula of the Institute (principle) of the Society. They are known in the fields of education (schools, colleges, universities, seminaries, theological faculties), intellectual research, and cultural pursuits in addition to missionary work, giving retreats, hospital and parish ministry, promoting social justice and ecumenical dialogue.

The Society was founded by St. Ignatius of Loyola, who after being wounded in a battle, experienced a religious conversion and composed the Spiritual Exercises in order to help others more closely follow Christ. In 1534, Ignatius gathered six young men, including St. Francis Xavier and Bl. Pierre Favre, and together they professed vows of poverty and chastity, and then later, obedience, including a special vow of obedience to the Pope. Rule 13 of Ignatius' Rules for Thinking with the Church said: "I will believe that the white that I see is black if the hierarchical Church so defines it". Ignatius' plan of the order's organization was approved by Pope Paul III in 1540 by the bull containing the Formula of the Institute. The opening lines of this founding document would declare that the Society of Jesus was founded to "strive especially for the propagation and defense of the faith and progress of souls in Christian life and doctrine." The Society participated in the Counter-Reformation and later in the implementation of the Second Vatican Council in the Catholic Church.

The Society of Jesus is consecrated under the patronage of Madonna Della Strada, a title of the Blessed Virgin Mary, and it is led by a Superior General, currently Adolfo Nicolás. The headquarters of the Society, its General Curia, is in Rome. The historic curia of St Ignatius is now part of the Collegio del Gesù attached to the Church of the Gesù, the Jesuit Mother Church.

The Jesuits today form the largest single religious order of priests and brothers in the Catholic Church, although they are surpassed by the Franciscan family of first orders OFMs, Capuchins, and Conventuals. As of 1 January 2007, Jesuits numbered 19,216: 13,491 clerks regular (priests), 3,049 scholastics (students to become priests), 1,810 brothers (not priests) and 866 novices, serving in 112 nations on six continents with the largest number in India and USA. Their average age was 57.34 years; 63.40 years for the priests, 29.89 years for the scholastics and 65.54 years for the brothers. With regard to its administration, the Society is currently divided into 91 Provinces with 12 dependent Regions: three in Africa, four in the Americas and five in Asia and Oceania. All together, they constitute 10 administrative units. (Assistances).

The current Superior General of the Jesuits is the Spaniard Adolfo Nicolás. The Society is characterized by its ministries in the fields of missionary work, human rights, social justice and, most notably, higher education. It operates colleges and universities in various countries around the world and is particularly active in the Philippines and India. In the United States alone, it maintains over 50 colleges, universities and high schools. A typical conception of the mission of a Jesuit school will often contain such concepts as proposing Christ as the model of human life, the pursuit of excellence in teaching and learning and life-long spiritual and intellectual growth.

Ignatius lays out his original vision for the new order in the Formula of the Institute of the Society of Jesus (established principle), which is "the fundamental charter of the order, of which all subsequent documents were elaborations and to which they had to conform." He ensured that his formula was contained in two papal bulls signed by Pope Paul III in 1540 and by Pope Julius III in 1550. The formula expressed the nature, spirituality, community life and apostolate of the new religious order. Its famous opening statement echoed Ignatius' military background: "Whoever desires to serve as a soldier of God beneath the banner of the Cross in our Society, which we desire to be designated by the Name of Jesus, and to serve the Lord alone and the Church, his spouse, under the Roman Pontiff, the Vicar of Christ on earth, should, after a solemn vow of perpetual chastity, poverty and obedience, keep what follows in mind. He is a member of a Society founded chiefly for this purpose: to strive especially for the defence and propagation of the faith and for the progress of souls in Christian life and doctrine, by means of public preaching, lectures and any other ministration whatsoever of the Word of God, and further by means of retreats, the education of children and unlettered persons in Christianity, and the spiritual consolation of Christ's faithful through hearing confessions and administering the other sacraments. Moreover, he should show himself ready to reconcile the estranged, compassionately assist and serve those who are in prisons or hospitals, and indeed, to perform any other works of charity, according to what will seem expedient for the glory of God and the common good".

On August 15, 1534, Ignatius of Loyola (born Íñigo López de Loyola), a Spaniard of Basque origin, and six other students at the University of Paris – Francisco Xavier from Navarre (modern Spain), Alfonso Salmeron, Diego Laínez, Nicolás Bobadilla from Spain, Peter Faber from Savoy, and Simão Rodrigues from Portugal – met in Montmartre outside Paris, in a crypt beneath the church of Saint Denis, now Saint Pierre de Montmartre.

They called themselves the Company of Jesus, and also Amigos En El Señor or "Friends in the Lord", because they felt "they were placed together by Christ." The name had echoes of the military (as in an infantry "company"), as well as of discipleship (the "companions" of Jesus). The word "company" comes ultimately from Latin, cum + pane = "with bread", or a group that shares meals.

In 1537, they traveled to Italy to seek papal approval for their order. Pope Paul III gave them a commendation, and permitted them to be ordained priests. These initial steps led to the founding of what would be called the Society of Jesus later in 1540. The term societas in Latin is derived from socius, a partner or comrade.

They were ordained at Venice by the bishop of Arbe (June 24). They devoted themselves to preaching and charitable work in Italy, as the Italian War of 1535-1538 renewed between Charles V, Holy Roman Emperor, Venice, the pope and the Ottoman Empire rendered any journey to Jerusalem impossible.