Lepeophtheirus salmonis and Caligus elongatus are the two common species of sealice responsible for serious disease problems in salmonid aquaculture. L. salmonis in particular is the most serious parasitic infection on Atlantic salmon farms in the Northern Hemisphere and is the best-known species. This review examines the voluminous literature on the biology and control of sealice and brings together ideas for developing our knowledge of these organisms. Research on the distribution, host range, structure, life cycle, epidemiology, laboratory maintenance, reproductive biology, physiology and pathogenesis is reviewed in depth. The control strategy and economic cost to the industry is discussed. The interactions between wild and cultured salmonids are examined.