Suitable for amateur and professsional naturalists, this new Synopsis provides a concise account of the morphology, habits and distribution of 113 species of athecate hydroids and their medusae recorded from the north-east Atlantic. Keys to identification are included, and these use external characters where possible. Hydroids and their medusae are amongst the most exquisitely beautiful and intriguing of marine invertebrate groups – and the history of their science reads like a detective story. The 'normal' life cycle alternates between sexual, free-swimming medusae and asexual, benthic hydroids, but many variations exist within the group. It was many years before even the basic life cycle was recognised, so that hydroids and medusae were classified independently of each other. And the story is by no means over: several species are currently known only from their hydroid or medusae stage. This new Synopsis is the first work to provide descriptions of all the northern European athecate hydroids since Hincks (1868) and the first ever book to include keys to their identification. It is the first on anthomedusae since Russell (1953).