  Gamma-ray bursts are thought to be the outcome of a cataclysmic event leading
to a relativistically expanding fireball, in which particles are accelerated at
shocks and produce nonthermal radiation. We discuss the theoretical predictions
of the fireball shock model and its general agreement with observations. Some
of the recent work deals with the collimation of the outflow and its
implications for the energe tics, the production of prompt bright flashes at
wavelenghts much longer than gamma-rays, the time structure of the afterglow,
its dependence on the central engine or progenitor system behavior, and the
role of the environment on the evolution of the afterglow.
