  Optical variability in galaxies at high redshift is a tracer of evolution in
AGN activity, and should provide a useful constraint on models of galaxy
evolution, AGN structure, and cosmology. We studied optical variability in
multiple deep CCD and photographic surveys of blank fields for galaxies with
$B_j = 20 - 25$ mag. Weakly variable objects are far more common than strongly
variable ones. For objects near $B_j = 22$, $0.74\% \pm 0.2 \%$ vary by
0.026~mag RMS or more, over a decade. This is small compared with previous
claims based on photographic surveys, and also small compared with the fraction
of bright quasars ($\approx 5\%$ at $B_j = 20$~mag) or Seyferts ($\approx
1-2\%$ for $B_j < 18$). The fraction of objects that vary increases slowly with
magnitude. Detection probabilities and error rates were checked by simulations
and statistical analysis of fluctuations of sample sky spots.
