Not yet reviewed by Pith; the record is open.

We present an analytical approach to the dynamical evolution of fireballs or axially symmetric jets expanding into an external medium, with application to Gamma-Ray Burst remnants. This method leads to numerical calculations of fireball dynamics that are computationally faster than hydrodynamic simulations. It is also a very flexible approach, that can be easily extended to include more complex situations, such as a continuous injection of energy at the reverse shock, and the sideways expansion in non-spherical ejecta. Some features of the numerical results for the remnant dynamical evolution are discussed and compared to the analytical results. We find that the ratio of the observer times when the jet half-angle reaches twice its initial value and when the light cone becomes wider than the jet is substantially smaller than predicted analytically. The effects arising from the remnant curvature and thickness further reduce this ratio, such that the afterglow light-curve break due to the remnant finite angular extent overlaps the weaker break due to the jet's sideways expansion. An analysis of the effects of the curvature and thickness of the remnant on the afterglow light-curves shows that these effects are important and should be taken into account for accurate calculations of the afterglow emission.