1 Introduction

In the fireball/blast wave model, the impulsive release of energy by a cosmic source forms an expanding fireball whose energy is transformed into a relativistic blast wave, which attains an initial Lorentz factor $\Gamma_0\gt\gt 1$ determined by the baryon loading of the fireball (Blandford & McKee 1976; Rees & Mészáros 1992). The power-law behaviors of the X-ray and optical afterglows discovered as a result of the Beppo-SAX mission are simply explained as the Doppler-shifted nonthermal lepton synchrotron emissions (e.g., Katz 1994; Tavani 1997) from a decelerating blast wave (e.g., Waxman 1997; Vietri 1997).

Less attention has been devoted to using this model to understand the large body of hard X-ray/soft $\gamma$-ray phenomenology of GRBs. Here we summarize our recent work which shows how the phenomenology of GRBs can be understood in the context of a simple formula based on the blast wave model.