  Since their discovery in 1973, Gamma-Ray Bursts (GRBs) have remained for many
years one of the most elusive mysteries in High Energy-Astrophysics. The main
problem regarding the nature of GRBs has usually been the lack of knowledge of
their distance scale. About 300 GRBs are detected annually by BATSE in the full
sky, but only a few of them can be localized accurately to less than half a
degree. For many years, follow-up observations by other satellites and
ground-based telescopes were conducted, but no counterparts at other
wavelengths were found. The breakthrough took place in 1997, thanks to the
observation by BeppoSAX and RossiXTE of the fading X-ray emission that follows
the more energetic gamma-ray photons once the GRB event has ended. This
emission (the afterglow) extends at longer wavelengths, and the good accuracy
in the position determination by BeppoSAX has led to the discovery of the first
optical counterparts -for GRB 970228, GRB 970508, and GRB 971214-, greatly
improving our understanding of these puzzling sources. Now it is widely
accepted that most bursts originate at cosmological distances but the final
solution of the GRB problem is still far away.
