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Publisher Summary Fishes of the tropical region constitute one of the most fascinating subjects of study in comparative biology. The myriad radiations and adaptive modifications displayed by different groups of tropical fishes cover practically all important phenomena in evolutionary biology. In many instances, tropical fishes provide unique case studies for specific processes. Tropical fishes occupy practically all aquatic environments, either permanent or temporary, and it is nearly impossible to encounter a body of water in the tropics, however small or uninviting, where there are no fish living. They occur from high-mountain freezing streams to hot stagnant anoxic pools to subterranean waters. Across their wide range of habitats, tropical fishes display elaborate adaptations that adjust their mode of living to widely different and often extreme conditions. Historical factors, with their contingencies, are a primary factor determining net diversity and taxonomic composition of fish faunas. Ecological factors are explanatory only within the constraints posed by historical background, expressed as phylogenetic patterns coupled with geological information. A review of the fish faunas in different regions shows that the evolution of continents, river drainages, and ocean basins are tantamount in canalizing fish diversity. The physiological adaptations of tropical fishes are a biological attribute as dependent on evolutionary history as any other. Therefore, research on comparative physiology relies heavily on the results of phylogenetic investigations.