In the early Middle Ages, many religious houses followed the Benedictine Rule, while others developed their own patterns (see "Monastery Life" in Chapter 7). In the High Middle Ages, this diversity became more formalized, and religious orders, groups of monastic houses following a particular rule, were established. Historians term the foundation, strengthening, and reform of religious orders in the High Middle Ages the “monastic revival.”

In the period of political disorder that followed the disintegration of the Carolingian Empire, many religious houses fell under the control and domination of local lords. Powerful laymen appointed themselves or their relatives as abbots, took the lands and goods of monasteries, and seized monastic revenues. Accordingly, the level of spiritual observance and intellectual activity in monasteries and convents declined. The first sign of reform came in 909, when William the Pious, duke of Aquitaine, established the abbey of Cluny in Burgundy. Duke William declared that the monastery was to be free from any feudal responsibilities to him or any other lord, its members subordinate only to the pope. The monastery at Cluny, which initially held high standards of religious behavior, came to exert vast religious influence. In the eleventh century, Cluny was fortunate to have a series of highly capable abbots who ruled for a long time. In a disorderly world, Cluny gradually came to represent stability. Therefore, laypersons placed lands under its custody and smaller monastic establishments under its jurisdiction. In this way, hundreds of religious houses, primarily in France and Spain, came under Cluny’s authority.

Deeply impressed laypeople showered gifts on monasteries with good reputations. But as the monasteries became richer, the lifestyle of the monks grew increasingly luxurious. Monastic observance and spiritual fervor declined. Soon fresh demands for reform were heard, resulting in the founding of new religious orders in the late eleventh and early twelfth centuries.

The Cistercians (sihs-TUHR-shuhnz) best represent the new reforming spirit. In 1098, a group of monks left the rich abbey of Molesmes in Burgundy and founded a new house in the swampy forest of Cîteaux (see-TOH). The early Cistercians determined to keep their services simple and their lives austere, returning to work in the fields and other sorts of manual labor. As with Cluny, their high ideals made them a model, and 525 Cistercian monasteries were founded in the course of the twelfth century all over Europe. The Cistercians’ influence on European society was profound because they used new agricultural methods and technology and spread them throughout Europe. Their advanced agricultural approach brought wealth; however, wealth brought power. By the later twelfth century, as with Cluny earlier, economic prosperity and political power had begun to compromise the original Cistercian ideals.