Line defects in crystals are called dislocations. Dislocations had been considered in the context of the elastic continuum theory of solids, beginning with the work of Volterra, as a one-dimensional mathematical cut in a solid. Although initially viewed as useful but abstract constructs, dislocations became indispensable in understanding the mechanical properties of solids and in particular the nature of plastic deformation. In 1934, Orowan, Polanyi and Taylor, each independently, made the connection between the atomistic structure of crystalline solids and the nature of dislocations; this concerned what is now called an “edge dislocation”. A few years later, Burgers introduced the concept of a different type of dislocation, the “screw dislocation”. The existence of dislocations in crystalline solids is confirmed experimentally by a variety of methods. The most direct observation of dislocations comes from transmission electron microscopy, in which electrons pass through a thin slice of the material and their scattering from atomic centers produces an image of the crystalline lattice and its defects (see, for example, Refs.). A striking manifestation of the presence of dislocations is the spiral growth pattern on a surface produced by a screw dislocation. The field of dislocation properties and their relation to the mechanical behavior of solids is enormous. Suggestions for comprehensive reviews of this field, as well as some classic treatments, are given in the Further reading section.