The knowledge base for maritime health has a number of constant features that have become apparent over the last 150 years. These can be used to structure an analysis of the current state of knowledge and to identify where there is sound evidence about the nature and scale of risks and about the effectiveness of intervention to reduce harm. It can also show where there are deficiencies in knowledge and point to the ways in which these could be remedied. Past events, as discussed in the first article, also indicate the dynamics of the political, economic and human interactions that are central to improving knowledge and to its application to improve the health of seafarers. The sources of useful knowledge about seafarer’s health range from single case reports of an unusual disease to long-term studies of common chronic disease incidence. The most accessible events to record are clinically apparent illness, injury, or cause of death, but active investigative studies may look at risks in the environment, personal risk factors, or pre-clinical phases of disease. Comparisons between subsets of a population are needed to look rigorously at health risks or at the effectiveness of intervention. This is best done if information on the at risk population can be used as the basis for deriving the incidence or prevalence of illness and if the populations compared are as similar as possible in every way, except that being studied. Sometimes large studies in onshore populations can provide information that it is not feasible to collect on seafarers. Information on seafarers’ health can be collected in several settings: at sea, on arrival in port, during leave periods, or after retirement. For acute illness and for injury a single setting can provide the basis for estimating risks, but for chronic conditions cases arising in several settings have to be included and the at risk population calculated to enable the incidence to be studied. Knowledge about the health of seafarers can be used to improve prevention both by attention to the conditions of living and working at sea and by selection of seafarers who are considered ‘fit’ for work. It is also important for defining the needs for emergency care at sea and in port. The overall patterns of illness and injury in seafarers and how these compare with other workers are important inputs to regulatory decisions on the measures to be taken to reduce harm from illness and injury. Markers of improved seafarer health can confirm the effectiveness of measures taken with this goal in mind. Reducing the contribution of health-related impairment to accidents and other risks at sea requires knowledge of the effects of such impairments on performance and safety in the routine and emergency tasks of a seafarer. This information can then be used to determine whether someone with an impairment can safely work at sea.