Pug moths, a group of geometrid moths in the genera Eupithecia, Chloroclystis and Gymnoscelis, comprise perhaps the most difficult of the macrolepidoptera to identify with confidence. This work describes the biology and characters of all stages of the 52 species from Britain and Ireland, with individual maps showing their known distribution on a vice-county basis. The larvae are illustrated in detailed black-and-white drawings, and the adults in a series of colour photographs, arranged with species that are superficially similar juxtaposed. The genitalia of every species are also illustrated in figures drawn to show the main diagnostic features.