Microstructural evolution was studied quantitatively by electron backscattering diffraction in commercial purity Ti processed by high-pressure torsion (HPT) at room temperature. The results show that a heterogeneous microstructure develops during HPT processing with regions of both nanocrystalline grains (< 100 nm) and coarse grains (similar to 1-30 mu m). Tensile {10 (1) over bar2} twins were observed in the center of the disk after the first turn of HPT. The microhardness near the disk center increases with increasing HPT turns and the hardness after 5 turns is reasonably homogeneous at radial positions > 1 mm. The mechanism of grain refinement is characterized by dynamic recrystallization and the continuous formation of a necklace-like array of fine grains gradually consumes the larger grains in subsequent passes.