This chapter discusses the physical processes, by which O2 and CO2 move, within the soil-plant systems, and some factors that affect their concentrations and rates of movement. It also discusses the interrelation of soil aeration, with other soil properties, such as soil strength, hydraulic properties, and nutrient status. Measuring and characterizing the aeration status of soils in terms of plant behavior is sometimes called the “problem of soil aeration.” The importance of O2 resides in its role as the final electron acceptor in respiration. In some way, oxidative phosphorylation is coupled to the electron transfer chain and energy is conserved in the form of nucleotides, such as adenosine triphosphate (ATP) or high-energy intermediates. Oxygen controls respiration in the roots of intact plants as well as in individual cells or tissues. CO2 deficiency may limit the incorporation of NH4+ into amino acids. This may be due to limited formation of carbamyl phosphate that appears to be a major pathway for biological fixation of CO2 and NH4+, with energy from ATP.