  We present an analytical approach to calculate the hydrodynamics of the
interaction between a relativistic ejecta and a surrounding medium, whose
evolution serves as a model for Gamma-Ray Burst afterglows. We investigate the
effect of the relevant model parameters on the X-ray, optical and radio fluxes,
and the effect of a refreshed shock energy input and anisotropy in the ejecta
on the shape of the light curves. We compare our numerical results to observed
afterglows and give a quantitative description of the conditions (geometry and
physical parameters) in the ejecta that are compatible with the light curves of
the 970508 afterglow, for which a large number of accurate flux measurements
are available.
