Abstract Although the term defect may conjure up descriptions like “unusual” or “abnormal”, crystal defects are an ever-present and integral part of any crystal structure. In some cases, these structural and/or chemical heterogeneities are present at very low densities, but in many cases their abundance is large enough to cause profound deviations from nominal stoichiometry. Furthermore, many types of defects are not thermodynamically stable and thus affect the thermodynamic properties of the mineral in which they reside. Similarly, the process of unmixing (precipitation or exsolution) commonly produces both chemical and structural heterogeneity in solid solutions. It is therefore appropriate, perhaps essential, to consider crystal defects and mechanisms of exsolution in a volume devoted to rock-forming mineral solid solutions. Crystal defects are commonly classified according to their dimensionality. Point defects are zero-dimensional: although obviously of finite size, they do not extend for appreciable distances in any direction. Their associated strain fields may, however, extend well beyond the unit cell in which they reside ( Carpenter & Boffa Ballaran, 2001 ). Dislocations are one-dimensional, or line defects, which may traverse an entire crystal while being spatially restricted to the unit-cell scale in the other two dimensions. “Planar” defects are two-dimensional. In fact, the term is used loosely to describe not only defects that are truly planar, but also two-dimensional defects that curve and thus are not restricted to a single plane. Taken together, dislocations and planar defects are commonly called extended defects . This chapter will provide an introduction to extended defects, although a brief