GRB Jet Propagation Outside the Progenitor

The structure of GRB jets has important implications for their energybudget and event rate, as well as for the mechanism responsible forthe collimation and acceleration of the outflow. I review our currentunderstanding of the structure of GRB jets and their dynamics as they sweepup the external medium and decelerate. A good understanding of the jetdynamics is important in order to use afterglow observations to constrainthe jet structure and energy, as well as the external density and itsradial profile, and the micro-physical parameters of the afterglow shock. Iend by briefly commenting about the possibleimplications of recent early X-ray observations by Swift for the energybudget of GRBs for different jet structures.