  A spectroscopic analysis of the whole sample of H$\alpha$ Emission-Line
Galaxies (ELGs) contained in the lists 1 & 2 of the Universidad Complutense de
Madrid (UCM) objective-prism survey is presented. A significant fraction (59\%)
of star-forming galaxies with low ionization or high extinction properties has
been found. This kind of ELG is only incompletely detected in the blue or in
other ELG surveys. We have found evidence for evolution among some of the
different ELG classes. A comparison between the populations detected by Case,
Kiso, UM and UCM surveys is presented. We conclude that a deep H$\alpha$ survey
is better able to sample all the ages, evolutionary stages and luminosities of
current star-forming galaxies than other surveys using blue-emission lines or
colors. Finally, the luminosity and spatial distributions of the UCM galaxies
are determined. The contribution of the newly found current star-forming ELGs
adds new clues to galaxy evolution and has to be taken into account when trying
to consider the density of ELGs and total Star Formation Rate in the Universe.
