An overview is presented on the interactive parameters that influence fish susceptibility to infection. In particular, the importance of genetically determined resistance, stress, immunocompetency, and nutrition are discussed in relation to their influence on susceptibility of salmonids to infection with sea lice. It is suggested that these factors should be taken into account, together with other factors that determine infection intensity, such as the source and number of infective stages of sea lice, when devising programs for the management of wild and aquacultured salmonids.