Permanent and indefinite survival of plants in flooded soils depends primarily on adaptations that increase the supply of oxygen to tissues in anoxic soil and water, usually associated with features such as (i) increased tissue porosity, (ii) changes in tissue permeability to gases, and (iii) changes in the topology of root and rhizome architecture. Differences in the degree of development of these features contribute not only to survival and growth, but are also responsible for species zonation in relation to hydrological gradients, as will be shown with examples of a range of species differing in flooding tolerance. Maintaining species diversity in managed wetlands therefore involves hydrological conditions suitable for a variety of plants that differ in their flooding tolerance. The shallowest areas of wetlands, in which soils are waterlogged but there is little standing water, can support many species which have root aeration adaptations but are otherwise unspecialised for aquatic life. Permanent standing water is a much greater challenge for plants, and survival here is restricted to species with special adaptations to their oxygen transport physiology such as the development of pressurized gas flows in their aerenchyma. These close linkages between flooding tolerance and species distributions are key considerations for maintaining species diversity in wetlands.