Geologic investigations are essential during the exploration, economic assessment, and production of geothermal resources. This chapter reviews the environments where these resources occur and the geologic techniques used in their investigation, which depart from basic geologic techniques such as field mapping. While the local geologic setting and the presence of surface manifestations, young volcanic rocks, and large-scale faults can guide an exploration program, no template exists that can be universally applied to identify geothermal systems. Descriptions of resources from various environments are presented to illustrate their diversity and similarity, including unconventional geothermal resources like enhanced and advanced geothermal systems and supercritical resources.