In the age of sail, seamen struggled to survive not only the hazards of battle but daunting health problems brought on by faulty diets and long periods at sea in closely confined quarters. Ships that left port with banners flying all too often returned with remnants of a crew, haggard and sick and scarcely able to set a sail. As ships became more seaworthy, navigation instruments more reliable, and lengthy voyages more common, efforts to keep crews healthy became increasingly important. This study by a surgeon traces the history of medicine under sail and chronicles attempts by 'sea surgeons' to surmount overwhelming barriers to treat injuries and disease and to curtail epidemics. In an age of discovery and empire building the map of the world was drawn by those on long voyages. Their achievements had as much of an impact on world history as did the admirals' success in implementing tactics that won the battles for colonialism.