This book is a companion volume to the World Atlas of Marine Fishes. With over 8,200 of relatively large and beautifully presented color photographs it provides the greatest possible coverage of selected groups of the enormous variety of marine invertebrates in a single book. It is primarily aimed to assist people who have a keen interest in the small animals they encounter in the sea other than fishes, to find their scientific names or where to classify them. Presented here are the species one is likely to see when diving or snorkeling in the sea, as well as those that are traded in the aquarium industry, with photographs large enough to see details for their identification. Most species occur on shallow reefs in tropical seas, with the greatest species diversity in the Southeast Asian regions. This World Atlas includes many of the familiar decapod, crustacea...lobsters, crabs, shrimps; some worms and the highly diverse mollusca...sea-slugs and some other snails such as cowry shells shown with their animal, as well as octopus and squid; and various other groups of mobile invertebrates such as worms, flatworms, sea stars, urchins and many other echinoderms. With the rare exception, the animals were all photographed in the wilderness. The popular groups, symbiotically living crustaceans and cephalopods are covered extensively, whilst the sea-slugs are covered as the second part, as the very large group of the colorful nudibranchs were comprehensively treated in our book Nudibranchs of the World, and thus expanded here with the other opisthobranch families such as head-shield slugs and sea-hares. To care for our oceans we need to know and have to show others what lives in this wet world. Today's live-forms are those which have successfully evolved and were shaped and created along their evolutionary path of hundreds of millions of years, and will keep on changing as required by circumstances. Our learning about the marine creatures is a never ending story, especially about their behavior. The World Atlas of Marine Fauna is an invaluable reference book for a great variety of marine invertebrates that we can admire in the world's oceans. As such, it addresses the environmental enthusiasts, photographers, sports divers, snorkelers, surveyors, teachers, and biogeographers (scientists as well as amateurs or hobbyists) and people at all ages.