Emission line galaxies are the most easily detected and studied objects in the high redshift Universe. They are being used to trace the evolution of critical observables of the Universe such as Star Formation Rate densities, starburst properties and abundances. Most of the research is being done using [OII]3727 and UV lines, but H α is still one the best tracers for Star Formation Rate and physical properties of current star-forming galaxies. As a complementary contribution to studies of galaxy evolution, our team has focused into a long-term project to study the population of H α -selected star-forming galaxies of the Universe at different redshifts. In 1995 we first determined the local H α luminosity function, and from it the Star Formation Rate density (SFRd) of the local Universe. We then, using narrow-band imaging in the optical, extended this measurement to z 0.24 and z 0.4. Working in the near-infrared, GTC will be a very powerful tool to study the evolution of the H α emission-line galaxy populations at different redshifts. We will both quantify the SFRd evolution and characterize the star-forming galaxy populations by directly comparing the same observables at all redshifts up to z 2.5.