The monastic military orders were among of the most distinctive manifestations of the crusading movement. The earliest were the Knights Templar. They began as a small group of warriors protecting pilgrims travelling to Jerusalem in the years after the First Crusade (1095–99). Over time they grew to become both a vast international operation and a formal institution of the Church. Later years saw the establishment of many other military orders, following their example. These included the Hospitallers, a medical and charitable institution formed prior to the Templars, but which acquired a military role in the 1120s. Another was the Teutonic Knights, a medical and later military order established in 1190 during the Third Crusade. These would become the three largest military orders and they all rose to prominence at least in part through their dedication to the defense or reconquest of Jerusalem. Elsewhere, from the twelfth century onwards, many other military orders emerged to fight Christendom's opponents, especially in Iberia and the Baltic.

In their origins, most military orders, including the Templars, Hospitallers and Teutonic Knights, were small operations. They conducted their vocations with the assistance of local patrons or visiting pilgrims who expressed their support by offering buildings, income, or land to facilitate their activities. In some cases, the smaller military orders never grew much beyond this simple model. Conversely, some military orders expanded to become massive international institutions and, in each case, they brought about this growth by capturing the attention of patrons in Western Christendom.