Gamma-ray Bursts: 15 Years of GRB Afterglows

This is a brief review of some recent progress in our understanding of GRB jet dynamics, during the early acceleration phase and the later afterglow phase. In the acceleration phase I focus on the possible role of impulsive magnetic acceleration, and its ability to convert most of the initial magnetic energy into kinetic energy and naturally produce efficient internal shocks at mild magnetizations. For the afterglow phase I outline new generalized yet simple analytic models that finally agree with numerical simulations, and present recent simulation results for a jet propagating into a stratified external medium.