© Springer-Verlag Wien 2014. The parts played by oxygen diffusion (and in some species, convected ‘air’) in facilitating aerobic metabolism in plants subject to soil flooding and submergence are explored. Simple diffusion equations are used to illustrate how resistance and respiration interact to create oxygen gradients and experimental and modelling examples of gradients in roots and the limitations of diffusive transport are presented and discussed. Attention is drawn to the limiting effects of diffusion especially in non-wetland species such as Arabidopsis thaliana. Here, a paucity of root gas-space and a cortical cell configuration found also in crop species, such as pea, tomato and cotton, is particularly unsuited for long-distance oxygen transport. The contrast with other more flood-tolerant Brassicas is highlighted. The relative roles of root aerenchymas and barrier formation to radial oxygen loss in improving oxygen supply and supporting root extension and phytotoxin exclusion in flooded soils are considered. Methods for monitoring radial oxygen loss from roots and oxygen concentrations both within and external to the plant are discussed, as are the results of analogue and more complex mathematical models that predict and explain the role of both diffusive and convective transport in plant aeration. Finally, pressurized gas-flow mechanisms and their ability to overcome diffusion limitations are briefly described and discussed.