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.

Expanded version of a review to appear in Prog.Theor.Phys. S.136 (1999), "Black Holes and Gravitational Waves - New Eyes in the 21st Century", Yukawa International Seminar'99, eds. T. Nakamura et al. Uses ptpsty.tex, ptp-text.sty (included), 21 pages, 6 figures