Gas can have a significant effect on the behaviour of soils, so that it is important to be able to identify its occurrence. This paper describes the forms in which gas can occur and the consequences for the selection of a suitable conceptual model. In a sand or stiff soil, gas typically displaces pore water without changing the overall structure. In contrast, in a soft soil, gas exists in voids that are large by comparison with the soil particles and the water-filled voids. Measurements of pore pressures can be used to determine the structure of a soil containing gas and, coupled with an appropriate choice of theoretical model, can be used to provide an indication of gas content. Examples are taken from laboratory and field tests.