  We simulate the dynamics of the interaction between relativistically
expanding spherically symmetric shells using a 1-dimensional hydrodynamic code
and calculate spectra and light-curves arising from such collisions by
integrating the synchrotron and inverse Compton emission of the shocked gas.
The numerical results reflect the most important features observed in Gamma-Ray
Bursts: the spectrum exhibits a progressive softening (its break energy
decaying exponentially with the 50--300 keV photon fluence), and the pulses
that form the burst appear narrower in higher energy bands. Analytical results
for the most important physical parameters of the burst are obtained by solving
the shock jump conditions for a pair of interacting shells, in the case when
both the forward and reverse shocks are relativistic.
