  We have performed searches for isolated flares and for steady flickering in
Gamma Ray Burst light curves on the microsecond to millisecond time scales. Two
bursts out of our sample of 20 revealed four isolated flares with time scales
from $256 \mu s$ to $2048 \mu s$. A wavelet analysis for our sample showed low
level flickering for all bursts on time scales from $256 \mu s$ to 33 ms, with
the majority of bursts containing rise times faster than four milliseconds and
30% having rise times faster than one millisecond. These results show that
millisecond variability is common in classical bursts and not some exceptional
activity by a possibly separate class of bursts. These fast rise times can be
used to place severe limits on burst models: (1) The characteristic thickness
of the energy generation region must be less than 1200 km along the line of
sight. (2) The angular size of the gamma ray emission region as subtended from
the central source must be less than 42 arc-seconds. (3) The expanding ejecta
must have a range of Lorentz factors along a radius line with a dispersion of
less than roughly 2%. (4) Within the external shock scenario, the
characteristic dimension of the impacted cloud must be smaller than 16
Astronomical Units on average. (5) Within the collimated jet scenario, the
collimation angle must be smaller than 42 arc-seconds.
