Gamma-Ray Bursts (GRBs) are the brightest objects observed. They are also the most relativistic objects known so far. GRBs occur when an ultrarelativistic ejecta is slowed down by internal shocks within the flow. Relativistic particles accelerated within these shocks emit the observed gamma-rays by a combination of synchrotron and inverse Compton emission. External shocks with the circumstellar matter slow down further the ejecta and produce the afterglow, which lasts for months. Comparison of the predictions of this fireball model with observations confirm a relativistic macroscopic motion with a Lorentz factor of Γ⩾100. Breaks in the light curves of the afterglow indicate that GRBs are beamed with typical opening angles of a few degrees. The temporal variability of the gamma-rays signal provide us with the best indirect evidence on the nature of the “internal engine” that powers the GRBs and accelerates the relativistic ejecta, suggesting accretion of a massive disk onto a newborn black hole: GRBs are the birth cries of these black holes. Two of the most promising models: Neutron Star Mergers and Collapars lead naturally to this scenario.

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