Lepeophtheirus salmonis (Atlantic) has become an important model for the study of ectoparasitic infestations on salmon. The process of disease due to L. salmonis on Atlantic salmon (Salmo salar) and sea trout (S. trutta) is a result of their feeding behavior and the secretion of bioactive compounds into the host. Clinical signs include physical damage to the epidermis and underlying tissue, changes in the blood electrolyte composition, physiological stress, immune dysfunction, loss of locomotive capacity and sometimes, death. Innate resistance to the salmon louse varies among the various species of salmon and trout. In addition, the question arises as to whether separate Atlantic and Pacific populations of L. salmonis have co-evolved with Atlantic salmon and Pacific salmon respectively. Recent data suggests that the Pacific Ocean population may be a distinct group from the Atlantic Ocean population. The salmon louse, Lepeophtheirus salmonis, is a common parasite of the skin of farmed and wild salmon throughout the northern hemisphere. In farming, the parasite is important because of production losses and significant costs of treatment. Furthermore, the salmon louse is notable for the debates that disease is transmitted between farmed and iconic wild salmon populations.