The growth and deformation of salt crystals are governed by, and in turn cause, defects in the crystal structure. A detailed study of these defects might therefore yield considerable information on the history of a crystal and the formation in which it occurs. This paper presents some examples of the study of these lattice defects and their application to geological problems. The growth of salt crystals from brine is extremely sensitive to slight variations in growth conditions. The resultant veils and bands of impurity ions and/or color centers may therefore be used as “microvarves” to correlate periods of simultaneous deposition and, when used together with trace-element analyses, brine analyses, and other defect studies, to determine the conditions of deposition. Salt crystals exhibit a wide range of mechanical properties which depend on the defect structure and prior history of each individual specimen. Nonetheless, most of these idiosyncrasies can be explained in terms of dislocation phenomena. These considerations have also led to a partial understanding of the behavior of polycrystalline salt. The ductility of salt is greatly enhanced at elevated temperatures or by the presence of water. The mechanical and thermal history of a salt deposit may, to some extent, be revealed by a study of the etch-pit patterns and birefringence of its component crystals; however, room-temperature annealing and artifacts introduced during sampling can lead to serious misinterpretations. The study of blue halite crystals, which frequently occur in potash deposits, may be of considerable significance, inasmuch as the absorption spectrum and patterns of coloration can be interpreted in terms of growth bands, slip bands, fracture, recrystallization, and thermal history. Many of these techniques of investigation are time consuming or expensive. Moreover, there is the possibility that the results so obtained may be too localized to permit extrapolation to the over-all geological system. Nonetheless, there are undoubtedly instances in which these crystal-defect studies will form a valuable adjunct to conventional field methods.