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This chapter provides a historical perspective for the development and implementation of mobile gamma ray spectrometry techniques. The chapter also focuses on the state of the art of in situ gamma spectrometry (IGS) and airborne gamma spectrometry (AGS) systems, perhaps the two end members of environmental gamma spectrometry, although similar systems have been deployed in road vehicles. IGS and AGS systems were originally fundamental geophysical tools used in geological mapping of large areas, primarily in the search for enriched uranium deposits and often in combination with other geophysical instrumentation. IGS was often used as a tool for overcoming issues of sampling error when undertaking mapping or prospecting survey work. Influences of the vertical activity distribution generally have a greater impact on IGS measurements than AGS measurements because of the larger solid angle between the IGS detector and source, unless the detector is collimated. Nevertheless, solutions to providing independent means of measuring changes in the vertical activity distribution through IGS are becoming more commonly used.