This work studies the deformation mechanisms active in two pure hexagonal close packed metals, beryllium (Be) and zirconium (Zr), during equal channel angular extrusion processing. An experimental-theoretical approach is employed to assess their relative contributions through measurement and calculation of texture evolution. A new multi-scale constitutive model, incorporating thermally activated dislocation density based hardening, is shown to effectively predict texture evolution as a function of processing route, number of passes (up to four), initial texture, pressing rate, and processing temperature. Texture predictions are shown to be in very good agreement with experimental measurements. Also, it is found that the two most active deformation modes in Be are basal slip and prismatic slip, where the predominant one is interestingly found to depend on die angle. Deformation in Zr during the first pass is predicted to be accommodated not only by its easiest mode, prismatic slip, but by basal slip and tensile twinning.