  Gamma Ray Bursts (GRBs) are unpredictable and brief flashes of gamma rays
that occur about once a day in random locations in the sky. Since gamma rays do
not penetrate the Earth's atmosphere, they are detected by satellites, which
automatically trigger ground-based telescopes for follow-up observations at
longer wavelengths. In this introduction to Gamma Ray Bursts we review how
building a multi-wavelength picture of these events has revealed that they are
the most energetic explosions since the Big Bang and are connected with stellar
deaths in other galaxies. However, in spite of exceptional observational and
theoretical progress in the last 15 years, recent observations raise many
questions which challenge our understanding of these elusive phenomena. Gamma
Ray Bursts therefore remain one of the hottest topics in modern astrophysics.
