Radio Loud and Quiet Gamma-ray Bursts.

 We present recent results on the dichotomy of long GRBs with and without radio afterglows, showing that the two populations exhibit significantly different properties: those with a radio radio afterglow are significantly longer in prompt (gamma-ray) duration, are more energetic in isotropic energy emitted, show the presence of very high (>10 GeV) gamma-ray emission, and possibly exhibit an anti-correlation between prompt duration and redshift.  We discuss these results in terms of GRB progenitor systems. In addition, we put constraints on magnetic fields and black hole masses of both short and long gamma-ray bursts assuming a Blandford-Znajek process powers the GRB jet, and discuss the implications this has on the properties of their progenitors.