Dislocations in minerals have a long history: The first images of dislocations ever seen were obtained by Seidentopf in 1905 on rocksalt (halite) with an optical microscope. Since then, and particularly after the development of ion thinning techniques in the early 1960s, dislocations in minerals have been studied in great detail by transmission electron microscopy. While diffraction contrast images of dislocations are similar in metals and minerals, the structures of most minerals are very complex and of low symmetry, which leads to their having a great variety of slip systems that change with temperature-pressure-strain rate conditions. Thus the identification of dislocations in rock-forming minerals and the analysis of their characteristics can be used to infer geological conditions during deformation. Recently much emphasis has been placed on the nature of dislocations in high-pressure phases that occur deep in the Earth. Here findings from transmission electron microscopy are combined with atomic scale modeling. This chapter provides an overview of early and modern work on dislocations in minerals and discusses applications to different mineral groups and their geologic significance.