The cap model is based on classical incremental theory of work hardening plasticity. It was developed in order to represent the important characteristics (i.e. compaction, shear, hysteresis, failure) of the behavior of soils, rocks, and concrete in the plastic region. The cap model has been used to study various geotechnical problems, such as ground shock effects from explosive sources, and soil-structures problems, including buried pipeline, retaining walls, foundations, embankments, in-situ tests, etc. Good results have been found using the cap model. DiMaggio and Sandler, 1971were the first to introduce the cap model, since then several versions of cap models have been successfully developed. Most of these versions of cap models have been implemented in many specialized finite element codes, as well as general purpose finite element codes, such as DYNA-3D, and ADINA. In this paper a general review of the cap model, highlighting the features and shortcomings of each version, is presented. Also the effects of various parameters that form the yield surface and the hardening cap, on two of the most recent versions of the cap model, (Simo et al version, 1988, and General Atomic version, 1989) are evaluated using the general purpose finite element code DYNA-3D by modeling uniaxial strain tests on soil, rock, and concrete materials.