Gamma Ray Bursts emit 1051-1054erg, mostly in Gamma rays around 1MeV. The short timescale variability (down to a ms) implies a very compact source, and therefore high photon density. With these conditions, the optical depth to electron positron pair creation is huge, in contradiction with the observed non thermal spectrum. The only way out of this “compactness problem” is extreme relativistic motion. We show that the sources of Gamma Ray Burst must emit relativistic flows, with Lorentz factor exceeding several hundreds. We describe the generic model, “the fireball model”, for Gamma Ray Bursts, which is independent of the inner engine that initiated the phenomenon. We argue, that in order to efficiently produce variability the source itself must be variable, and the emission should takes place in internal shocks, within the relativistic flow. We discuss the implication to possible inner engines, such as binary neutron stars merger or exploding massive stars.