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Published online by Cambridge University Press: 22 July 2013
GRB’s environment can be studied through the imprint (i.e. absorption)it leaves on the afterglow emission. Fast optical observations allowed us to observe linevariability and to derive the distance and composition of the absorbing medium. However,given the huge GRB ionizing flux, this medium starts showing up in the optical spectraonly at relatively large distances (100 pc). A complementary view comes from the X-rayband, where the overall contribution of all metals bends the power law X-ray afterglowspectrum. We will review and compare optical and X-ray studies of the circumbustmedium.