Deep-sea hydrothermal vents are singular in character among habitats on Earth. Unlike most other ecosystems, in which the photosynthetic activities of green plants provides the energy for living organisms, the ecosystems of hydrothermal vents are driven by chemosynthetic micro-organisms. These chemosynthetically-based ecosystems are biodiversity "hot spots" – colonized by a high proportion of endemic species – and are insular habitats. The deep-sea ecosystems are also largely decoupled from anthropogenic effects and the contemporary climate but are, however, strongly coupled to planetary properties of plate tectonics and volcanism. Strung like pearls along the mid-ocean ridges that girdle the globe, hydrothermal systems are expressions of the interaction between seawater and basalt. Despite the similarity in habitat and dispersal throughout the oceans, species found at the vents are not globally cosmopolitan, nor is species richness globally uniform. One can look to tectonic history and volcanic periodicity as fundamental predictors of the boundaries of biogeographic provinces. In such putatively simple systems, controls on biodiversity and biogeography – such as hydrography and topography – may be much more readily resolved than in terrestrial systems where climate and human activities obscure the processes that control biodiversity.