It is now recognized that processing by equal-channel angular pressing (ECAP) leads to very significant grain refinement in polycrystalline materials with the as-pressed grains typically having sizes within the submicrometer range. Furthermore, the materials produced by ECAP exhibit many useful properties including a high strength at ambient temperatures and, if these ultrafine grains are retained to elevated temperatures, a potential for superplastic forming. This paper examines the fundamental characteristics of grain refinement by making use of two sets of experimental observations: experimental data obtained from the pressing of aluminum single crystals through one pass of ECAP and hardness measurements taken on polycrystalline aluminum for samples subjected to ECAP for up to a total of eight passes. These experimental results are used to develop a microstructural model that provides a satisfactory explanation for the grain refinement occurring in ECAP.