The second completelty revised Edition of the “Handbook of Hydrothermal Vent Fauna” gives on overview of our current knowledge on the animals living at hydrothermal vents. The discovery of hydrothermal vents and progresses made during almost 30 years are outlined. A brief introduction is given on hydrothermal vent meiofauna and parasites. Geographic maps and a table of mid-ocean ridges and back-arc basins with the major known hydrothermal vent fields, their location and depth range and the most prominent vent sites are provided. Higher taxa are presented individually with information on the current taxonomic and biogeographic status, the number of species described, recommendations for fixation, and schematic drawings, which aim to help non-specialists to identify the animals. 81 authors contributed with their expertise to create a comprehensive database on animals living at hydrothermal vents, which contains information on the morphology, biology, and geographic distribution of more than 500 currently described species belonging to one protist and twelve animal phyla. It comprises also the largest collection of more than 1000 pictures of hydrothermal vent animals taken in situ with submersibles, in vivo after collection, and with various dissection, light, and scanning electron microscopes after fixation and preparations.