  Despite the pre-Swift expectation that bright optical flashes from reverse
shocks would be prevalent in early-time afterglow emission, rapid response
observations show this not to be the case. Although very bright at early times,
some GRBs such as GRB 061007 and GRB 060418, lack the short-lived optical flash
from the reverse shock within minutes after the GRB. In contrast, other optical
afterglows, such as those of GRB 990123, GRB 021211, GRB 060111B, GRB 060117,
GRB 061126, and recently GRB 080319B, show a steep-to-flat transition within
first 10^3 s typical of a rapidly evolving reverse + forward shock combination.
We review the presence and absence of the reverse shock components in optical
afterglows and discuss the implications for the standard model and the
magnetization of the fireball. We show that the previously predicted optical
flashes are likely to occur at lower wavelengths, perhaps as low as radio
wavelengths and, by using the case of GRB 061126 we show that the magnetic
energy density in the ejecta, expressed as a fraction of the equipartion value,
is a key physical parameter.
