This paper presents a methodology for evaluating the environmental hazards posed by gas migration from oil and gas reservoirs, or underground natural gas storage facilities, into the near-surface environment. Faults, fractured zones, and improperly completed or abandoned wellbores (e.g., due to poor cementing practices) are described as the primary pathways by which the gas can reach the surface. Furthermore, the gas migration problem can be exacerbated by subsidence, earthquake activity, and well corrosion. Soil gas monitoring, geochemical gas fingerprinting, and geological profiling are used in order to identify the magnitude and location of the environmental risks. Shallow and deep soil probes are used in order to define the near-surface hydrology, and to identify possible collector zones where gas concentrations can build up to dangerous levels. These techniques have proven to be important in the planning for and design of mitigation systems necessary to protect residential and commercial properties from the migrating gases. For example, some jurisdictions have imposed regulatory controls and design requirements regarding the installation of gas mitigation systems. Safe operating procedures must be established for the operation of oil and gas fields, and underground natural gas storage facilities. A number of case histories are discussed here that have been used by the authors to validate the methodology, and to illustrate the seriousness of the problem. A clear case is made for the need to perform ongoing monitoring for these conditions, especially in an urban setting.