Sea louse (Family Caligidae: genera Caligus and Lepeophtheirus) infection of farmed salmonids represents a significant threat to animal welfare and undermines profitability. Lice may also act as vectors for the transmission of viral and bacterial pathogens. Pest-control programmes parallel those deployed in terrestrial livestock farming and include the use of parasiticides. The authorisation process for fish medicines varies widely between salmon farming countries and undue regulatory constraint may place farmers in one country at a competitive disadvantage. In many jurisdictions, fish are a 'minor' species and mounting demands for environmental assessment increase registration costs. A successful integrated louse-management strategy requires free access to a range of effective, chemically unrelated active ingredients deployed according to current best practice. Over-reliance on a limited number of products will lead, inevitably, to resistance, which is difficult to counter.