Early on 1997, the field of Gamma Ray Bursts had a dramatic breakthrough. The Italian-Dutch satellite, BeppoSAX, delivered accurate positioning of several events. Dozens of ground based and space based observatories monitored the given position, and found decaying emission in x-ray, optical and radio, lasting for years after the events. These observations, established the distance scale to the explosions and confirmed much of the theoretical understanding given by the fireball model. The afterglow is produced once the relativistic flow, which earlier produced the burst itself, interact with its surrounding. Shock waves are produced, in which particles are accelerated and then emit synchrotron and inverse Compton radiation. As more and more of the surrounding matter is accumulated, the flow decelerates, and the radiation shifts to lower and lower frequencies. We review the theory of the afterglow emission, and compare it with recent observations. We show, that such comparisons are able to uncover the properties of the burst environment, and therefore provide clues for the inner engine.