  The afterglow emission has become the main stream of Gamma-Ray burst research
since its discovery three years ago. With the distance-scale enigma solved, the
study of the late-time GRB emission is now the most promising approach to
disclose the origin of these explosions and their relationship with the
environment of the host galaxy in the early phase of the Universe. In this
contribution I will review X-ray observations and their implication on our
undertstanding on the GRB phenomenon. These measurements are providing a direct
probe into the nature of the progenitor and a measurement of the GRB beaming
properties, crucial to establish the total energy output. Some evidence of iron
lines connects the GRB explosion with massive progenitors, thence with
star-forming regions. Furthermore a comparison of the spectral properties with
the temporal evolution indicates that the fireball expansion should not be - on
average - highly collimated, with a jet angle $>10 \deg$.
