The use of imposed plastic deformation as a single parameter to compare results of samples processed by severe plastic deformation is not always accurate. Therefore, this report describes the theoretical plastic flow occurring during high-pressure torsion and presents finite element modeling of this technique to complement the theory. The results demonstrate the influence on plastic flow of the material behavior, the sample aspect ratio, the processing pressure, and the contact friction between the sample and the anvil. It is shown that heterogeneous flow is primarily observed near the edges of the samples. The present results are in general agreement with published experimental observations.