The synthesis of organic molecules in hydrothermal fluids raises the question of what role hydrothermal vents may have played in the emergence of life on Earth. These deep‐sea ecosystems were most likely protected from extreme conditions at the surface (meteor showers and ultraviolet radiation). Since the middle of the 19th century, scientists have questioned the position of deep‐sea animals in the evolution of marine fauna, in the hope to discover the missing link, i.e. the species or group that may have survived mass extinctions in this protected and stable environment. This chapter discusses the geology and geochemistry of hydrothermal systems, microbial chemosynthesis, symbioses and trophic chains, and fauna at different spatial scales. Fauna assemblages colonize narrow mixing gradients between hydrothermal fluids and seawater, on hard substrataor in thick sediment layers. Most temporal studies are not based on continuous temporal monitoring but on regular visits by manned or unmanned submersibles.