The aquatic Hemiptera include a few inland semiaquatic Homoptera, but overwhelmingly belong to the Heteroptera. The aquatic and semiaquatic Heteroptera, consisting of the infraorders Leptopodomorpha, Gerromorpha, and Nepomorpha, comprise a significant component of the world's aquatic insect biota. Within these three infraorders as a whole there are 24 families, 343 genera, and 4799 species group taxa considered valid, of which 21 families, 323 genera, and 4656 species inhabit inland waters. In addition, more than 1100 unequivocally diagnosed species remain to be described. Aquatic Heteroptera occur on all continents except Antarctica, and are most numerous in the tropical regions, although there are many distinctly cold adapted genera. Overall species richness is highest in the Neotropical and Oriental regions, which harbor 1289 and 1103 species, respectively. In comparison with these core tropical regions, species richness is significantly lower in the Afrotropical (799 species), Australiasian (654 species), Palearctic (496 species), Nearctic (424 species), and Pacific (37 species) regions. Aquatic Heteroptera are notable for utilizing an exceptionally broad range of habitats, from marine and intertidal to arctic and high alpine, across a global altitudinal range of 0–4700 m. Species may be found in almost every inland aquatic biotope, and many exhibit striking morphological adaptations to their aquatic environment, making them excellent subjects for ecological and biogeographic studies.