  Most supernova explosions accompany the death of a massive star. These
explosions give birth to neutron stars and black holes and eject solar masses
of heavy elements. However, determining the mechanism of explosion has been a
half-century journey of great complexity. In this paper, we present our
perspective of the status of this theoretical quest and the physics and
astrophysics upon which its resolution seems to depend. The delayed
neutrino-heating mechanism is emerging as a robust solution, but there remain
many issues to address, not the least of which involves the chaos of the
dynamics, before victory can unambiguously be declared. It is impossible to
review in detail all aspects of this multi-faceted, more-than-half-century-long
theoretical quest. Rather, we here map out the major ingredients of explosion
and the emerging systematics of the observables with progenitor mass, as we
currently see them. Our discussion will of necessity be speculative in parts,
and many of the ideas may not survive future scrutiny. Some statements may be
viewed as informed predictions concerning the numerous observables that rightly
exercise astronomers witnessing and diagnosing the supernova Universe.
Importantly, the same explosion in the inside, by the same mechanism, can look
very different in photons, depending upon the mass and radius of the star upon
explosion. A 10$^{51}$-erg (one "Bethe") explosion of a red supergiant with a
massive hydrogen-rich envelope, a diminished hydrogen envelope, no hydrogen
envelope, and, perhaps, no hydrogen envelope or helium shell all look very
different, yet might have the same core and explosion evolution.
