Evolution of massive stars is determined by several important physical processes that we still do not fully understand theoretically. They include stellar wind mass loss, binary interactions, and stellar rotation. Interestingly, these three primary factors for the evolution of massive stars are closely related to production of hydrogen-deficient massive stars. The best known example is Wolf-Rayet stars: it is believed that the hydrogen-rich envelopes of their progenitors have been stripped off via stellar winds. Binary interactions and rotationally-induced chemical mixing can also make important contribution to the production of hydrogen-deficient stars. Wolf-Rayed stars and other forms of hydrogen-deficient stars (e.g., helium stars in massive binary systems) may produce a number of ionizing photons, and eventually explode as core-collapse supernovae of Type IIb, Ib and Ic or as long gamma-ray bursts depending on their final structure at the pre-collapse stage. Observing these stars and their explosions would therefore provide excellent constraints for stellar evolution theory. In this talk, I will review evolutionary scenarios of massive stars towards hydrogen-deficient stars, and their confrontation with recent observations.