The possibility to detect the emission line of interstellar neutral hydrogen at 21 centimeters wavelength revolutionized astronomy in the second half of the 20th century, as a powerful tool to explore the structure and evolution of galaxies and the star formation within them. A second revolution came, when, at millimetre wavelengths, molecular hydrogen could be detected and measured in directly using, above all, the emission from the lines of the CO molecule. Surprisingly, the emission from the third phase of interstellar hydrogen, the ionized phase, which is at optical wavelengths, has had less attention. The basic aim of GHaFaS is to make up for lost time by taking H-alpha velocity fields of galaxies at high spatial and spectral resolution.