A material is said to have deformed plastically if it doesn’t return to its original shape after the load is removed. The most common application for plasticity in nite element analysis is modeling the deformation of metals. While a great deal of research is available on the plasticity of metals at a very fundamental level, the majority of plasticity models are phenomenological and have no motivation beyond duplicating experimental data. The models discussed here are among the simpler plasticity models, and they are, indeed, completely phenomenological. While there are post hoc rationalizations of why these simple models perform as well as they do, they are still rationalizations.