The relativistic ejecta expelled in a gamma ray burst explosion drives a strong blast wave that propagates in the circumburst medium. This blast wave produces the afterglow emission that follows the prompt phase. Observations seem to indicate strong amplification of magnetic fields in the shocked circumburst shell by some unknown mechanism. In addition, it has been anticipated that the reverse shock sweeping the ejecta at early times should produce optical flashes. However, such flashes seem to be rare. In this talk, after reviewing the observational motivation, I'll discuss how hydrodynamic instabilities, particularly the relativistic Rayleigh-Taylor instability, affect the evolution and structure of the blast wave, and speculate that it can lead to a strong amplification of magnetic fields and change the properties of the emission from the reverse shock.