'Sea lice' is a common name for a large number of species of marine ectoparasitic copepods, many of which are widespread and important disease-causing agents that infect both cultured and wild fish. Of these copepods, the salmon louse Lepeophtheirus salmonis is the most extensively studied because of its economic impact on the salmonid aquaculture industry and its possible impacts on wild salmonid populations. Different levels of infection by this parasite can affect the long-term survival and viability of its hosts. In this article, we review the nature of the interactions between L. salmonis and it hosts to identify crucial areas that warrant further research to aid understanding of the impact of infection with L. salmonis.