Most of the Gemini flights carried a terrain photography experiment, whose objective was to obtain small-scale color photographs, with 70 mm hand-held cameras, of selected land and ocean areas for geologic, geographic, and oceanographic research. The experiment was highly successful, with nearly 1,100 usable pictures being returned. This paper describes the terrain photography experiment, presents specific geologic applications of the Gemini pictures, and discusses the advantages and disadvantages of geologic orbital photography. Equipment used for the terrain photography included Hasselblad 500-C and SWC cameras, and the Maurer 70 mm Space Camera, with 38 mm, 80 mm, and 250 mm focal length lenses. Most pictures were taken with Ektachrome; also used were Anscochrome D-50 and Ektachrome Infrared film. Haze filters were used with the Hasselblad cameras. Specific geologic applications of the Gemini photographs include: ( 1) study and remapping of part of northern Baja California, Mexico, in the vicinity of the Agua Blanca fault; ( 2) study of the tectonics and geology of northern Chihuahua, Mexico, in the vicinity of Palomas; ( 3) unsuccessful search for the Texas Lineament; ( 4) study of a regional dune-fracture pattern surrounding the Tibesti massif in Chad and Libya; and ( 5) study of the theory that the Arabian Sea is a sphenochasm, formed by continental drift. The major advantages of orbital photography, as shown by the Gemini pictures, appear to be large area per picture, global coverage, unlimited dissemination, availability of color or multispectral coverage, and wide range of scales. Major disadvantages, compared to aerial photography, include difficulty of changing the flight path and of obtaining high-latitude coverage, the necessity for continual spacecraft orientation, high degree of global cloud cover, daylight restrictions, resolution limits, target acquisition, film degradation due to radiation, loss of resolution and color rendition from atmospheric scattering, and the necessity of vertical or near-vertical camera orientation. All of these problems can be overcome if mission planning and spacecraft design allow fully for photographic requirements. It is concluded that the geologic value and feasibility of orbital photography have been demonstrated. The most promising geologic uses of orbital photographs appear to be in regional geologic mapping, tectonics, sedimentation, planning of geological and geophysical field work, and interpretation of regional geophysical surveys.