Spectra of SN 1993J at three epochs within the first 1.5 months after the explosion reveal the onset of a remarkable transformation. The prominent Halpha emission line visible at early times became progressively weaker as P Cygni features of He I gradually appeared; SN 1993J is therefore a hydrogen-poor, helium-rich ''Type IIb'' supernova. The progenitor of SN 1993J was probably a reasonably massive star (initial mass 10-17 M.) that transferred most, but not all, of its hydrogen envelope to a physically bound companion. We predict that many (greater than or similar to) months past maximum brightness, the spectrum of SN 1993J will closely resemble the late-time spectra of Type Ib supernovae; it will be dominated by strong emission lines of [O I], [Ca II], and Ca II, with Halpha very weak or absent. A similar metamorphosis has been clearly seen only in SN 1987K; however, since the development of He I lines could not be monitored, SN 1987K may have actually been Type IIc rather than Type IIb. SN 1993J demonstrates that SN 1987K was not a unique event. It strengthens the hypothesis that Type Ib and Ic supernovae are not white dwarfs undergoing thermonuclear runaway. Rather, they are triggered by core collapse in massive, evolved stars that have lost their outer envelope of hydrogen (Ib) and perhaps helium as well (Ic), often through mass transfer in close binary systems. Thus, these objects are fundamentally related to Type II supernovae.
