INTRODUCTION Gas is encountered underground in such a variety of geologic environments that few could be categorically declared gas-free. The presence of gas is easiest to explain when the environment in question is actively generating gas. But genera tion is only one aspect of gas distribution. Gas can become separated in space or in time from its origin, so explaining its presence in a particular geologic setting becomes more complicated. This chapter describes forces that control movement, retention, and temporal existence of natural gases in soils, rocks, and groundwater.