The Knights Hospitaller, also known as the Order of Hospitallers or simply Hospitallers, were a group of men attached to a hospital in Jerusalem that was founded by Blessed Gerard around 1023 which evolved into a military/hospitaller order.

The Hospitallers arose because of the work of a Amalfitan hospital located in the Muristan district of Jerusalem, founded around 1023 to provide care for poor, sick or injured pilgrims to the Holy Land. After the Western Christian conquest of Jerusalem in 1099 during the First Crusade, the organisation became a religious and military order under its own charter, and was charged with the care and defence of the Holy Land. Following the conquest of the Holy Land by Islamic forces, the Order operated from Rhodes, over which it was sovereign, and later from Malta where it administered a vassal state under the Spanish viceroy of Sicily.

The Order was weakened by Napoleon's capture of Malta in 1798 and became dispersed throughout Europe. It regained strength during the early 19th century as it repurposed itself toward humanitarian and religious causes. In 1834 it was renamed the Sovereign Military Order of Malta, with its headquarters in Rome. The heads of all five modern Orders of St. John assert that the SMOM is the original order and that four Protestant orders stem from the same root. Protestant branches of the Order exist in Germany, the Netherlands and Sweden, and as a British revival headquartered in London.