INTRODUCTION Hazardous gas incidents in civil works excavations involve so many variables that patterns may seem indiscernible, making meaningful extrapolation appear impossible. On the contrary, gas occurrences in the near-surface underground follow consistent geological patterns, and the consequences of gas in an excava­ tion relate to the effectiveness of atmosphere control measures. This chapter iden­ tifies common geologic environments that may contain all the factors necessary to pose a gas risk to construction. It describes environments in which bacterial, thermogenic, and geothermal gases are generated, migrate, are retained, and can enter excavations.