This investigation has been mainly confined to providing descriptions for the identification of the various winged morphs of the adelgid species occurring in Britain. Morphological keys and descriptions, together with 32 figures, are given for the determination of the 14 winged morphs of the 9 species having alatae. In addition, a key to the adult apterae of the genus Pineus is included to enable identification to be made of those species which do not have a winged form. Only in the cases of Adelges viridis and A. abietis gallicolae, and Pineus orientalis and P. pini sexuparae, is it still necessary to depend upon life cycle information for separation down to species. Three of the species described, A. viridana, P. pineoides and P. orientalis, have not previously been recognised in Britain. Details are given of transfer experiments carried out to check host alternation pattern in the alatae of certain species. A world check list of adelgids, their common hosts and distribution is appended.