Gamma Ray Burst Beaming Constraints from Afterglow Light Curves: GRB 970508

The beaming angle (zeta) is the main uncertainty in gamma ray burst energy requirements today. We summarize predictions for the light curves of beamed bursts, and model the R band light curve of GRB 970508 to derive zeta > 30 degrees. This yields an irreducible minimum energy requirement of 3.4 times 10^{49} ergs to power the afterglow alone.