Gas and water in soils interact through the physical laws of Boyle, Henry, Darcy and Fick and through the capillarity law; this interaction has a large number of practical implications and applications in geotechnical engineering. Those examined in this paper are: cavitation; implications of occluded gas; implications of gas exsolution on hydraulic conductivity, pore pressures and sampling; gas hydrate dissociation; saturation of soil samples; infiltrations in soils; capillary barriers; and transient pore pressures in earth dams.