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.