The Order of Cistercians of the Strict Observance (O.C.S.O.: Ordo Cisterciensis Strictioris Observantiae), or Trappists, is a Roman Catholic religious order of cloistered contemplative monks who follow the Rule of St. Benedict. The Trappists are a branch of the Order of Cistercians and like the other Cistercian orders, they have a women's branch, commonly referred to as the Trappistines.

The order takes the name of "Trappist" from La Trappe Abbey or La Grande Trappe in Normandy in France. A reform movement began there in 1664, in reaction to the relaxation of practices in many Cistercian monasteries.Armand Jean le Bouthillier de Rancé, originally the commendatory abbot of La Trappe, led the reform. As commendatory abbot, de Rancé was a layman who obtained income from the monastery but had no religious obligations. After a conversion of life between 1660 and 1662, de Rancé formally joined the abbey and became its regular abbot in 1663. In 1892 the reformed "Trappists" broke away from the Cistercian order and formed an independent monastic order with the approval of the Pope.

Thomas Merton, O.C.S.O. (January 31, 1915 – December 10, 1968) was an Anglo-American Catholic writer and mystic. A Trappist monk of the Abbey of Gethsemani, Kentucky, he was a poet, social activist, and student of comparative religion. In 1949, he was ordained to the priesthood and given the name Father Louis.

Merton wrote more than 70 books, mostly on spirituality, social justice and a quiet pacifism, as well as scores of essays and reviews, including his best-selling autobiography, The Seven Storey Mountain (1948), which sent scores of World War II veterans, students, and even teen-agers flocking to monasteries across the US, and was also featured in National Review's list of the 100 best non-fiction books of the century. Merton was a keen proponent of interfaith understanding. He pioneered dialogue with prominent Asian spiritual figures, including the Dalai Lama, the Japanese writer D.T. Suzuki, and the Vietnamese monk Thich Nhat Hanh. Merton has also been the subject of several biographies.