2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/7444I will review the constraints set by X-ray measurements of afterglows on several issues of GRB, with particular regard to the fireball model, the environment, the progenitor and dark GRB.Invited talk at the 4th workshop on Gamma-Ray Bursts in the Afterglow Era, ASP Vol.312, in pressAstrophysicstext