The most important metazoan parasites of farmed Atlantic salmon are the sea lice Lepeophtheirus salmonis and Caligus elongatus. Adults of these caligid copepods are responsible for serious damage to the skin of salmon, which may die unless treated with organophosphorus pesticides. Alan Pike presents what is known of the biology of these important pathogens and highlights the shortcomings of this knowledge in terms of useful insights which could lead to more effective control measures.