Progenitors of gamma-ray bursts

The last four years have seen a revolution in the field of gamma-ray burst astronomy. With the detection of long-lived counterparts in the X-ray, opti cal, and radio bands, we have now firmly established the distance scale to these explosive events, we have a basic physical understanding of the after glow emission, and it is generally believed that GRBs signal the formation of a black hole. Observational evidence is emerging that the progenitors of Iona duration events are massive stars, however this is yet to be firmly e stablished. A number of observational techniques hold great promise for the near future, and it is likely that within the next decade we will understa nd the properties of both the GRB sources and their environments. This revi ew concentrates on our understanding of the progenitors, and those observat ional techniques best-suited to make progress on this question. This review , accompanying a talk given by SRK at the 2000 Nishinomiya conference, has two biases: it emphasizes the unique contributions of radio afterglow measu rements, and it details in particular the work of the Caltech-NRAO-CARA GRB collaboration.