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We examine, from a theoretical viewpoint, how the physical parameters of HIIregions are controlled in both normal galaxies and in starburst environments.These parameters are the HII region luminosity function, the time-dependentsize, the covering fraction of molecular clouds, the pressure in the ionizedgas and the ionization parameter. The factors which control them are theinitial mass function of the exciting stars, the cluster mass function, themetallicity and the mean pressure in the surrounding interstellar medium. Weinvestigate the sensitivity of the H$\alpha$ luminosity to the IMF, and findthat this can translate to more than a factor two variation in derived starformation rates. The molecular cloud dissipation timescale is estimated from acase study of M17 to be $\sim1$ Myr for this object. Based upon HII luminosityfunction fitting for nearby galaxies, we suggest that the \HII region clustermass function is fitted by a log-normal form peaking at $\sim 100 M_{\odot}$.The cluster mass function continues the stellar IMF to higher mass. Thepressure in the HII regions is controlled by the mechanical luminosity fluxfrom the central cluster. Since this is closely related to the ionizing photonflux, we show that the ionization parameter is not a free variable, and thatthe diffuse ionized medium may be composed of many large, faint and old HIIregions. Finally, we derive theoretical probability distributions for theionization parameter as a function of metallicity and compare these to thosederived for SDSS galaxies.Comment: 36 pages, 8 figures. To be published in Astrophysical Journal, August  200

Modeling the Pan–Spectral Energy Distribution of Starburst Galaxies. II. Control of the HiiRegion Parameters