  The standard model of GRB afterglows assumes that relativistically expanding
material is decelerating due to interaction with the surrounding medium. The
afterglows are well described by the synchrotron radiation from a forward
shock, while the strong optical flash associated with GRB 990123 can be
attributed to the emission from a reverse shock. We give a detailed study on
the reverse shock emission. The full light curves are calculated for a long and
a short GRB cases. We discuss the lack of the prompt optical detections by
ROTSE for GRB 981121 and GRB 981223.
