Gamma-Ray Bursts environment and progenitors

The properties of the ambient medium in which GRBs go off are an important piece of the puzzle, not only as an issue in itself, but because of their link with the nature of the progenitor. In this review, I describe and critically comment the various experiments proposed to study the burst environment. I discuss emission and absorption features techniques and compare their results to afterglow modelling. A consistent picture cannot be drawn, and I suggest that the evaporation of the soft X-ray absorbing column is the more promising of the tools, even though the fast reaction of Swift is required in order to obtain data of sufficiently high quality.