We present the scientific analysis of the GRB afterglow data obtained by the GROND, a seven-channel imager with four optical and three near-infrared detectors. Its unique capability to observe in all bands simultaneously, together with rapid triggering, precise photometry and high temporal resolution, give us the opportunity to study light curves and spectral energy distributions of GRB afterglows in unprecedented detail. This is demonstrated using the observations of three GRB afterglows that are used to put strong constrains on the standard GRB fireball scenario.