A quantitative metallographic study of the effects of mechanical twinning on the uniaxial stress-strain curves of Cu-3.1Sn, Cu-4.9Sn, Cu-30Zn, and Co-40Ni has shown that twinning produces identifiable deformation stages in these alloys. These deformation stages are (1) an interval without twinning, (2) twinning on a single crystallographic plane within a grain, (3) twinning on intersecting twinning planes, and (4) dynamic recovery. In Stage 2, in which twins apparently form on the primary slip plane, twinning decreases the work hardening rate, whereas in Stage 3 the development of intersecting twin arrays increases the work hardening rate by progressively decreasing the effective grain size. The increased work hardening in Stage 3 accounts for the unusually high ductility of those alloys that twin heavily. The strength of the effects of the twinning on the stress-strain curve increases with increasing solute concentration and decreasing temperature.