  Observations of late-time optical bumps have been reported for several GRBs.
The timescale for such bumps, and colors of such when available, find a natural
explanation as due to associated supernovae. Ground-based and HST observations
of the afterglow and bump of GRB 011121, in particular, place the strongest
constraints yet on the physical nature of the supernova and any alternative
explanations, such as supranovae or dust echoes. I summarize the search for
underlying bumps in other GRBs and make the case for the supernova hypothesis
in light of observed bumps and bump non-detections (e.g., GRB 010921).
