This book keys 92 of over 130 species of Orthocladiinae. An extensive programme of rearing larvae of the subfamily Orthocladiinae was undertaken by the author between 1975 and 1979. Individual larvae from 50 sites throughout Britain were reared and over 1200 emerging adults identified. Larval generic diagnoses, generic keys, species descriptions and, where possible, species keys were prepared from this identified associated material. Published descriptions and examinations of unreared larvae, including those of the Thienemann collection in Munich, were also incorporated. Ecological information was obtained for each species considered and this information is summarized in the present publication.