  GRBs have remained a puzzle for many high-energy astrophysicists since their
discovery in 1967. With the advent of the X-ray satellites BeppoSAX and
RossiXTE, it has been possible to carry out deep multi-wavelength observations
of the counterparts associated with the GRBs just within a few hours of
occurence, thanks to the observation of the fading X-ray emission that follows
the more energetic gamma-ray photons once the GRB event has ended. The fact
that this emission (the afterglow) extends at longer wavelengths, has led to
the discovery of the first optical/IR/radio counterparts in 1997-99, greatly
improving our understanding of these sources. Now it is widely accepted that
GRBs originate at cosmological distances. The observed afterglow satisfies the
predictions of the "standard" relativistic fireball model, and the central
engines that power these extraordinary events are thought to be the collapse of
massive stars or the merging of compact objects.
