  We use recent observations of high-redshift galaxies to study the evolution
of galactic disks over the redshift range 0<z<1. The data are inconsistent with
models in which disks were already assembled at z=1 and have evolved only in
luminosity since that time. Assuming that disk properties change with redshift
as powers of 1+z and analysing the observations assuming an Einstein-de Sitter
universe, we find that for given rotation speed, disk scalelength decreases
with z as ~(1+z)^{-1}, total B-band mass-to-light ratio decreases with z as
~(1+z)^{-1}, and disk luminosity (again in B) depends only weakly on z. These
scalings are consistent with current data on the evolution of disk galaxy
abundance as a function of size and luminosity. Both the scalings and the
abundance evolution are close to the predictions of hierarchical models for
galaxy formation. If different cosmogonies are compared, the observed evolution
in disk-size and disk abundance favours a flat low-density universe over an
Einstein-de Sitter universe.
