  In this paper we review the present status and implications of H-alpha
surveys at various redshifts. With the advent of sensitive wide-format
near-infrared detectors on large telescopes, deep and extensive H-alpha surveys
are now feasible to redshift z~2.5. The cosmic star formation history can
therefore be traced out to this redshift using H-alpha alone, avoiding
complications arising from the comparison of different tracers at different
redshifts. The H-alpha surveys to date confirm the rapid increase in luminosity
density from z=0 out to z=1, and show that this increase flattens off at higher
redshifts, remaining approximately constant out to at least z~2.2. We also
discuss the prospects for determining the masses of high redshift galaxies
based on emission lines. A set of high-quality H-alpha rotation curves of
samples of disk galaxies at a number of different redshifts would allow a study
of the evolution of the Tully-Fisher relation and address fundamental issues in
disk galaxy formation. Such a program remains challenging even with present-day
large telescopes.
