We present four lines of evidence leading to the conclusion that the HII regions in the discs of normal galaxies,notably the highly luminous regions,are density bounded: (1)The relation of central Hα surface brightness to total luminosity of a region departs from the predictions of uniformly ionization bounded systems (2)The geometry and intensity of the diffuse Hα from whole discs is well modeled if the Lyman continuum causing it leaks from the HIi regions (3) The Hα luminosity function of complete HII region populations shows a break whose parameters are explicable on the density bounding hypothesis and not on rival hypotheses,(4)The observed relation between the Hα velocity half-width and luminosity for HII regions is naturally accounted for via density bounding.A fractally clumpy cloud structure,and a straightforward law relating the mass of the most luminous star in a young cluster to the mass of its placental gas cloud give model parameters which can account for these observations.The parameters apply to conditions in galactic discs,and may well not apply directly to circumnuclear starbursts,but the fraction of ionizing photons which can escape from the whole galaxy can still be highly significant. We show how to quantify the fraction of the Lyman continuum escaping from leaky HII regions which finally escapes from the galactic discs,and can ionize large volumes of ultra-low density intra-cluster gas.We explore the possibility that this mechanism played a significant role during the reionization epoch of the early universe.