Recent studies of illness and injury in seafarers and of disease risk factors have been mapped. There is a good knowledge base on some aspects of health, especially on causes of death. By contrast there are very few studies on aspects of current importance, such as illness at sea, the scope for its prevention, and its treatment and outcome. Results are presented in terms of the settings in which the investigations were conducted: medical fitness examinations at recruitment and periodically, illness and injury at sea, telemedical advice, evacuation and urgent port referrals, repatriations, illness at other times in serving seafarers, health related cessation of work, and illness after cessation of work. Mortality studies were mapped in a similar way, as were population-based surveys of health and of risk factors. The scope for valid extrapolation of the results from studies in other populations to seafarers is discussed. A more immediate problem of extrapolation relates to the current knowledge base, which is largely derived from own nationality seafarers of the traditional developed world maritime nations. It is uncertain whether this can be validly extrapolated to seafarers from the major crewing countries, who come from populations with very different patterns of illness. Existing studies mirror the priorities of those who commissioned them, in that many of the most valid ones relate to the overall lifetime risks of seafaring in developed countries. These enable comparisons to be made with other occupational groups. The major concerns of many interest groups in the maritime sector about health are now focused on the risks within a single contract period and how these can most efficiently be minimized. Studies on this are limited in scope, are of uncertain validity, and are often used for operational purposes rather than entering the scientific literature. Gaps in knowledge about health risks over a relatively short timescale in seafarers from the major crewing countries have been identified, and the uncertainties about extrapolating from studies in traditional maritime nations to the majority of the world’s seafarers means that a major redirection of effort is needed if maritime health practice is to have a sound knowledge base on illness and injury risks in the future.