  We present numerical simulations of Gamma-Ray Bursts arising from external
shocks in the impulsive and wind models, including a weak or a strong coupling
between electrons and protons plus magnetic fields, and analyze the burst
features in each scenario. The dynamics of the ejecta and external medium are
followed into the late stages of deceleration, in order to study the
hydrodynamics of the remnant and the temporal and spectral evolution of the
afterglow. A brief comparison with the optical and radio afterglows of GRB
970228 and GRB 970508 is made.
