This history of the Sovereign Military and Hospitaller Order of St John of Jerusalem, Rhodes and Malta, also known as the Knights Hospitaller, is intended as an introduction to the Order. Beginning with a consideration of the origins of the Order as a hospice for pilgrims in Jerusalem in the eleventh century, it traces the Hospitallers' development into a military order during the first part of the twelfth century, and the Order's military activities on the frontiers of Christendom in the eastern Mediterranean, Spain and eastern Europe during the middle ages and into the early modern period: its role in crusades and in wars against non-Christians on land and at sea, as well as its role in building and maintaining fortresses.

The Order's activities away from the frontiers of Christendom are also considered: its economic activities and its relations with patrons and rulers throughout Europe, as well as its hospitaller work and its religious life. The main emphasis of the study is on the medieval period down to the loss of Rhodes in 1522, but the final chapters of the book consider the Order's history on Malta from the sixteenth to the end of the eighteenth century, and from the loss of Malta in 1798 to the present day.