Sea lice (primarily Lepeophtheirus salmonis and Caligus spp.) are ectoparasitic copepods causing significant welfare and economic harm in Atlantic salmon aquaculture. Heavy infestations cause scale loss, skin erosion, stress, secondary infection, and mortality. Treatment options include chemical baths (hydrogen peroxide, azamethiphos), thermal delicing (Thermolicer), freshwater treatments, biological control via cleaner fish (wrasse and lumpfish), and laser removal (Stingray). Each treatment modality carries welfare implications: chemical baths cause physiological stress; Thermolicer causes fin erosion and cataracts at incorrect temperatures; cleaner fish themselves suffer significant welfare problems. Integrated pest management combining surveillance and rotational treatment is current best practice.