  The optical luminosity function is a fundamental characterization of the
galaxy population. A combination of earlier redshift surveys with two new
surveys allows the first accurate determination of the evolution of the
luminosity function with redshift, and reveals a marked steepening of the faint
end slope. This effect is more profound for star-forming galaxies - there are
5-10 times as many star-forming galaxies at z~0.5 as there are locally. These
results, together with high-resolution imaging and linewidth velocity
measurements, support the view that the excess of star-forming galaxies at
moderate redshift represents a general increase in the star-formation rate of
normal galaxies rather than a distinct new population. This increase in
star-formation appears in lower-L galaxies at lower redshift and only appears
in L* galaxies at z>0.5. Imaging studies provide indirect evidence which
suggests that interactions are responsible for a large part of this increased
activity.
