It is a common misconception that fish health risk is negligible in RAS because they do not require the same inputs as other rearing systems and are thus protected from many of the infectious threats that come with the use of natural resources. RAS can support a level of biosecurity well beyond what is possible in open aquaculture. Some believe that adhering to a strict biosecurity plan is enough to protect their fish and investment from pathogens and the associated losses. Biosecurity is an essential element of fish health management, but to suggest that risk is minimal in biosecure RAS is to reveal a narrow view of fish health and a fundamental misunderstanding of risk assessment. Assuming environmental conditions are stable , pathogens are perhaps the most imminent threat to fish health in open aquaculture. As a result, fish health management in this context is rightly focused on the prevention, diagnosis, and treatment of infectious disease. However, in RAS-based aquaculture, a greater emphasis on physical injuries and environmentally or nutritionally driven diseases may be warranted. Of course, this doesn't mean that infectious disease is irrelevant is RAS. S ome pathogens are ubiquitous and resist even the most rigorous attempts to avoid or eradicate them. Risk is a function of both the likelihood of an event occurring and the severity of the consequences should the event come to pass. Although RAS operators can strive to minimize one or both of these factors, there is no such thing as 'zero risk'. This presentation will provide an introduction to risk assessment as it applies to fish health management in RAS, highlight a few pathogens of concern in salmonid aquaculture, and underscore the need for vigilance and comprehensive fish health management in RAS culture of salmonids .