Equal-channel angular pressing (ECAP) may be used to achieve very significant grain refinement in bulk materials. Typically, the materials processed by ECAP have ultrafine grain sizes within the submicrometer range. This paper examines the development of the as-pressed microstructural characteristics during the pressing of pure aluminum single crystals and polycrystalline aluminum. Experimental results show that a homogeneous ultrafine-grained microstructure evolves with increasing numbers of passes through the ECAP die. These experimental observations are used to develop a model that provides a satisfactory explanation for grain refinement during processing by ECAP.