This work provides a practical guide to the external morphology and classification of the mesostigmatic mites of Britain and Ireland. In the account of the external morphology, particular attention is paid to those features which figure prominently in modern classifications of the Mesostigmata, such as, the structure of the gnathosoma, the ontogenetic development of the chaetotaxy of the idiosoma and legs, and the form of body sclerotization. The Order Mesostigmata is represented by five suborders in the fauna of Britain and Ireland; namely, Gamasina, Antennophorina, Microgyniina, Sejina and Uropodina. Keys, accompanied by illustrations, are provided for the identification of suborders, families and genera. A new genus Cornigamasusgen. nov. (type: Gamasus coleoptratorum var. lunaris Berlese, 1882), family Parasitidae, is proposed. Aspects of the biology of the group are discussed under family or superfamily headings. A brief account is given of the methods of collecting, preserving and preparing mites for study.