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