The tropics are renowned for extraordinary species richness across the marine and terrestrial realms, holding more than three quarters of all species [1]. Within the tropics, certain ‘hyperdiverse’ regions host truly exceptional numbers of species at the local community level. For example, a single hectare (100m x 100m) of coral reef in the Indo-Pacific Coral Triangle hosts twice as many coral species as the entire Atlantic ocean [2], and a hectare of the Ecuadorian Amazon holds more tree species than the entirety of Europe [3,4]. Causes of such outstanding species richness in hyperdiverse regions are varied, and in some cases enigmatic, but in many ways the forces structuring such high local-scale diversity have parallels across land and sea. These range from the heightened accumulation of species due to quirks of biogeography, climate and geology at evolutionary timescales, through to the pressures of living at close quarters with many competitors and natural enemies at ecological timescales. Comparing processes that assemble hyperdiverse tropical communities across the marine and terrestrial realms may allow future research to draw generalities about the evolution of hyperdiverse regions as a whole, and may illuminate ways in which they can be conserved.