The paper aims at introducing the reader to the principal theories of plasticity. Since a presentation of the general stress‐strain relations used in these theories would require too much space, the discussion is restricted to the mechanical behavior of plastic materials under shear. Theories of plastic deformation (Hencky, Nadai) and theories of plastic flow (Saint Venant‐Le´vy‐Mises, Prandtl‐Reuss, Prager) are illustrated by the example of a prismatic bar under torsion.