The early modern era saw important changes in the character of warfare in Europe, including the development of larger, permanent armies and navies. Historians have studied many key aspects of what some call the “military revolution”, whose character and timing have become a matter of debate; but some important features of these emerging military communities remain largely unexplored. One subject which has not attracted the attention it merits is that of the health of soldiers and sailors and of medical provision in the new armies and navies. The issue has not been entirely neglected, either generally, or as it relates to specific states, but focused studies are rare. This is unfortunate, not least because of the importance attached to the issue of sickness and medical provision by contemporaries, and the value of medical provision as a sort of test case by which to measure the effectiveness of medical services and hence to contribute to the “military revolution” debate. For some historians the later seventeenth and eighteenth centuries saw the first significant efforts to develop a structure of military and naval hospitals; for others, however, the extent of illness and the inadequacy of medical support services before the French Revolutionary and Napoleonic era suggests that many states failed to meet the organizational challenge posed by the growth of standing armed forces in the sixteenth, seventeenth and eighteenth centuries. What follows is an investigation of the extent and nature of illness, and the effectiveness of medical provision in the armies and navies of one major player of the period, Spain in the reign of the last Habsburg, Charles II (1665–1700).