The pages in this text are about the history, science and practice of medicine at sea and in the field. The practical portion is written for those providing medical care in isolated environments of a ship, in the wilderness or after natural disasters where individuals with limited formal medical education may be pressed into duty. Similar circumstances have been common throughout history. Only in the last century has medical education been taught over several years. Before that, most medical care delivered at sea and on land was performed by men and women with little to no formal medical education, trained by apprenticeship. Armed with a competent medical textbook, some practical skills and a modern medicine chest, a person taken back in time to the Second World War would be recognized as an "eagle" among medical officers.