  The successful discovery of X-ray, optical and radio afterglows of GRB has
made possible the identification of host galaxies at cosmological distances.
The energy release inferred in these outbursts place them among the most
energetic and violent events in the Universe. They are thought to be the
outcome of a cataclysmic stellar collapse or compact stellar merger, leading to
a relativistically expanding fireball, in which particles are accelerated at
shocks and produce nonthermal radiation. The substantial agreement between
observations and the theoretical predictions of the fireball shock model
provide confirmation of the basic aspects of this scenario. Among recent issues
are the collimation of the outflow and its implications for the energetics, 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 afterglow.
