  The various stages of baryonic gamma-ray burst afterglow blast waves are
reviewed. These are responsible for the afterglow emission from which much of
our understanding of gamma-ray bursts derives. Initially, the blast waves are
confined to the dense medium surrounding the burster (stellar envelope or dense
wind), giving rise to a jet-cocoon structure. A massive ejecta is released and
potentially fed by ongoing energy release from the burster and a
forward-reverse shock system is set up between ejecta and ambient density.
Ultimately the blast wave spreads sideways and slows down, and the dominant
afterglow emission shifts from X-rays down to radio. Over the past years
significant progress has been made both observationally and
theoretically/numerically in our understanding of these blast waves, unique in
the universe due to their often incredibly high initial Lorentz factors of
100-1000. The recent discovery of a short gamma-ray burst counterpart to a
gravitational wave detection (GW 170817) brings the promise of a completely new
avenue to explore and constrain the dynamics of gamma-ray burst blast waves.
