  We discuss recent results on the ionizing fluxes of O stars obtained from our
"combined stellar structure and atmosphere models" (CoStar) accounting for
stellar winds, non-LTE effects and line blanketing. The implications on the
ionization structure of HII regions are summarized, and observational
constraints on the ionizing spectra and the total ionizing photon fluxes are
presented. Using our CoStar models we derive new consistent predictions for the
H and HeI ionizing fluxes of steady-state massive star populations.
Implications for the interpretation of observations of `Diffuse Ionized Gas' in
galaxies are discussed. Finally we present preliminary model calculations
aiming to improve our current predictions further and compare our results to
recent results from the Munich group.
