The Knights Hospitaller, also known as the Hospitallers, Order of Hospitallers, Knights of Saint John and Order of Saint John, are among the most famous of the Western Christian military orders during the Middle Ages.The Hospitallers probably arose as a group of individuals associated with an Amalfitan hospital in the Muristan district of Jerusalem, which is dedicated to St. John the Baptist and founded around 1023 by Blessed Gerard Thom to provide care for poor, sick or injured pilgrims to the Holy Land.

(Some scholars, however, consider that the Amalfitan order and Amalfitan hospital were different from Gerard's order and its hospital.)

After the Latin Christian conquest of Jerusalem in 1099 during the First Crusade, the organization becomes a religious and military order under its own Papal charter, and it is charged with the care and defense of the Holy Land.

Following the conquest of the Holy Land by Islamic forces, the Order operates from Rhodes, over which it is sovereign, and later from Malta where it administers a vassal state under the Spanish viceroy of Sicily.The Order is weakened in the Reformation, when rich commanderies of the Order in northern Germany and the Netherlands become Protestant (and, largely separated from the Roman Catholic main stem, remain so to this day), and the Order is disestablished in England, Denmark, and elsewhere in northern Europe.

The Roman Catholic order is further damaged by Napoleon's capture of Malta in 1798 and becomes dispersed throughout Europe.

It regains strength during the early 19th century as it redirects itself toward humanitarian and religious causes.

In 1834, the order, by this time known as the Sovereign Military Order of Malta (SMOM), acquires new headquarters in Rome where it has remained since.

As of 2013, the Roman Catholic order has about 13,500 members, 80000 volunteers, and 25000 mostly medical employees, and operates in about 120 countries across the world, including in Muslim nations; the Protestant branches of the order are smaller but engage in similar work.

Until recently the order focused mainly on developing countries, but following the introduction of austerity in the Eurozone and the United Kingdom which began in 2010, they have increasingly turned their attention to Europe, establishing shelters and soup kitchens to help the homeless and those suffering from hunger.

Five contemporary, state-recognized chivalric orders which claim modern inheritance of the Hospitaller tradition all assert that "The Sovereign Military Order of Malta is the original order" and that four non-Catholic orders stem from the same root: Protestant orders exist in Germany, the Netherlands and Sweden, and a non-denominational British revival is headquartered in the United Kingdom.