An observation based on long-standing experience is that a plastic body deformed under conditions which could result in a quasi-static, macroscopically uniform deformation process, in certain circumstances begins to deform in a quite different way. The term instability is commonly used in such cases though more in an intuitive than in a specified sense. Typical examples of so-understood instability are the buckling, bulging and necking, phenomena as “geometric” instabilities, snap-through phenomena as “dynamic” instabilities, and shear band localization of deformation as a “local” instability. Similar phenomena may naturally appear in the course of non-uniform deformations.