  Most gamma-ray bursts are made during the deaths of massive stars. Here the
environmental circumstances, stellar evolutionary paths, and explosion physics
that might produce the bursts are reviewed. Neither of the two leading models -
collapsar and millisecond magnetar - can be excluded, and both may operate in
progenitor stars of different masses, metallicities, and rotation rates.
Potential diagnostics are discussed and uncertainties highlighted. Both models
are capable of producing a wide variety of transients whose properties vary
with both stellar properties and viewing angle. Some of these are reviewed
including the possibility of very long (days) low luminosity bursts, so far
undiscovered, short hard bursts from massive stellar progenitors, and bursts
from very massive Population III stars.
