Thermal infrared images provide information about the near-surface physical state of geologic materials, particularly, the density, water content, and heat transfer. Nonterrestrial planetary studies, conducted at fairly coarse resolution, have been useful primarily in determining the distribution of rock fragments. Terrestrial studies, conducted from satellite and aircraft at coarse to fine resolutions, have been successful for monitoring effusive volcanism, delineating areas of steaming altered ground and hot-spring activity, detecting fractures expressed hydrologically and topographically, and distinguishing a variety of geologic materials with physical and compositional differences. Interpretation of thermal images is complicated by the various types of physical processes involved and commonly requires an assessment of many different factors. A simple theoretical model was used in this analysis to provide quantitative assessment of some of these factors, to predict optimum times to acquire thermal data, and to determine quantitative values of various properties of terrian. Two geologic applications were studied in some detail: geothermal mapping and thermal inertia mapping. Initial results indicate that both techniques have considerable potential, especially in reconnaissance studies. These data were acquired under optimal meteorological conditions and at sites where the geologic materials were well exposed. A realistic assessment of the limitations of these techniques must await future studies.