  A wide range of mechanisms have been proposed to supply the energy for
gamma-ray bursts (GRB) at cosmological distances. It is a common misconception
that some of these, notably NS-NS mergers, cannot meet the energy requirements
suggested by recent observations. We show here that GRB energies, even at the
most distant redshifts detected, are compatible with current binary merger or
collapse scenarios involving compact objects. This is especially so if, as
expected, there is a moderate amount of beaming, since current observations
constrain the energy per solid angle much more strongly and directly than the
total energy. All plausible progenitors, ranging from NS-NS mergers to various
hypernova-like scenarios, eventually lead to the formation of a black hole with
a debris torus around it, so that the extractable energy is of the same order,
1E+54 ergs, in all cases. MHD conversion of gravitational into kinetic and
radiation energy can significantly increase the probability of observing large
photon fluxes, although significant collimation may achieve the same effect
with neutrino annihilation in short bursts. The lifetime of the debris torus is
dictated by a variety of physical processes, such as viscous accretion and
various instabilities; these mechanisms dominate at different stages in the
evolution of the torus and provide for a range of gamma-ray burst lifetimes.
