The sea louse (plural sea lice) is a copepod within the order Siphonostomatoida, family Caligidae. There are around 559 species in 37 genera, including approximately 162 Lepeophtheirus and 268 Caligus species. Sea lice are marine ectoparasites (external parasites) that feed on the mucus, epidermal tissue, and blood of host marine fish.This article focuses on the genera Lepeophtheirus and Caligus which parasitize marine fish, in particular those species that have been recorded on farmed salmon. Lepeophtheirus salmonis and various Caligus species are adapted to saltwater and are major ectoparasites of farmed and wild Atlantic salmon. Several antiparasitic drugs have been developed for control purposes. Since L. salmonis is the major sea louse of concern and is the best understood in the areas of its biology and interactions with its salmon host, this review will focus on this species.Caligus rogercresseyi has become a major parasite of concern on salmon farms in Chile, and studies are under way to gain a better understanding of the parasite and the host-parasite interactions. Recent evidence is also emerging that L. salmonis in the Atlantic has sufficient genetic differences from L. salmonis from the Pacific, suggesting that Atlantic and Pacific L. salmonis may have independently co-evolved with Atlantic and Pacific salmonids, respectively.