  Observations show that at least some gamma-ray bursts (GRBs) happen
simultaneously with core-collapse supernovae (SNe), thus linking by a common
thread nature's two grandest explosions. We review here the growing evidence
for and theoretical implications of this association, and conclude that most
long-duration soft-spectrum GRBs are accompanied by massive stellar explosions
(GRB-SNe). The kinetic energy and luminosity of well-studied GRB-SNe appear to
be greater than those of ordinary SNe, but evidence exists, even in a limited
sample, for considerable diversity. The existing sample also suggests that most
of the energy in the explosion is contained in nonrelativistic ejecta
(producing the supernova) rather than in the relativistic jets responsible for
making the burst and its afterglow. Neither all SNe, nor even all SNe of Type
Ibc produce GRBs. The degree of differential rotation in the collapsing iron
core of massive stars when they die may be what makes the difference.
