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, optical, and radio bands, we have now firmly established the distance scale to these explosive events, we have a basic physical understanding of the afterglow emission, and it is generally believed that GRBs signal the formation of a black hole. Observational evidence is emerging that the progenitors of long-duration events are massive stars, however this is yet to be firmly established. A number of observational techniques hold great promise for the near future, and it is likely that within the next decade we will understand the properties of both the GRB sources and their environments. This review concentrates on our understanding of the progenitors, and those observational 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 measurements, and it details in particular the work of the Caltech-NRAO-CARA GRB collaboration.