Research work on unsaturated soils became very important during the last decade. A realistic numerical simulation of the three components of unsaturated soil as grains, water and gas and their interdependence is essential for economical design concepts for many problems in foundation engineering. Additionally for the decontamination of sites the method of soil-air-exhaust can be used. For all applications the knowledge of the flow of gases in soils is necessary. With large scale laboratory tests the behaviour of air in soils with regard to type of soil, density, degree of saturation, air permeability and pressure head is to be investigated and finally calculation methods are to be developed.