Cistercians, Roman Catholic monastic order founded in 1098 at Cîteaux (Latin Cistercium), France, by a group of Benedictine monks from the abbey of Molesme under the leadership of St Robert of Molesme. Also called White Monks because of the white or grey habit they wore under their black scapulars, the first Cistercians wanted to establish a community that would follow a strict interpretation of the monastic rules set forth by St Benedict of Nursia for his monks about 540. They favoured severe asceticism, manual labour as part of the monastic rule, and the rejection of feudal revenues. Although St Robert had been ordered by higher authority to return to Molesme in 1099, St Alberic, his successor at Cîteaux, succeeded in having the order approved by Pope Paschal II in 1100. The third abbot, St Stephen Harding, an Englishman, is credited with having formulated the Charter of Charity, the constitution of the Cistercian order. The order was to observe the same rules in all its houses which were to be visited annually by the founding abbot; all Cistercian abbots were to convene once a year at Cîteaux.

In 1113 St Bernard of Clairvaux joined the monastery of Cîteaux as a novice; in 1115 he became the founding abbot of Clairvaux. Ultimately the most influential spiritual leader of his time, he was largely responsible for the rapid expansion of the order. By 1153 there were more than 300 Cistercian monasteries, 68 of which were founded directly from Clairvaux. By the end of the Middle Ages more than 700 Cistercian abbeys existed; the order had spread through almost the whole of Europe and to the Levant.

As the order spread and prospered, new demands were placed upon it. To accommodate these demands the Cistercians departed from some of the ascetic ideals outlined in their constitution, partly because some of its provisions could not be reconciled with the order's spectacular growth. In the 12th century, considered their golden age, the Cistercians were the most influential order within the Roman Catholic Church. They took over the bishoprics and legatine offices that the Benedictines from Cluny had held in the 11th century, and replaced them in the Roman Curia, the governmental body of the Church. They also contributed substantially to economic life in the Middle Ages, particularly in the development of techniques for reclaiming wasteland and in the production and marketing methods for grain and wool. They influenced the spread of Gothic architecture throughout Europe and devoted considerable time to collecting and copying manuscripts for their libraries.

A period of decline after the 13th century was followed by a rise of new reformed groups of Cistercians. The most notable was the 17th-century group at La Trappe. Popularly called Trappists, this group eventually became a separate body, the Cistercians of the Strict Observance. The original order was then called the Cistercians of the Common Observance.