This, the first of two volumes on the Opisthobranchia, gives a general account of the sea-slugs and bubble-shells of the world's seas and concludes with a systematic account of the British representatives of the orders Bullomorpha, Aplysiomorpha, Pleurobranchomorpha, Acochlidiacea and Sacoglossa. The biology of the British Nudibrancha (the largest order of the Opisthobranchia) will be the subject of Volume II. The Opisthobranchia are popular with marine biologists and with general zoologists because of their common occurrence and their often vivid colours and elegant behaviour. They are a subclass of the gastropod molluscs, and all are hermaphrodite, marine and macroscopic. Worldwide there are about 3000 species, and around 150 of these have been recorded from the shallow waters around the British Isles. Of the British species, 30% are infaunal, the rest epifaunal. It is chiefly the epifaunal type, exemplified by the aplysiids or the nudibranchs, which has attracted the attention of naturalists for the last century and in the 1970s captured the interest of the growing body of aqualung enthusiasts. This volume is complementary to the pioneering monographs on the British nudibranchs published by the Ray Society in 1845-55 (by Joshua Alder and Albany Hancock) and in 1910 (by Sir Charles Eliot).