We build, as far as theory will permit, self-consistent model HII regions around central clusters of aging stars. These produce strong emission line diagnostics applicable to either individual HII regions in galaxies or to the integrated emission line spectra of disk or starburst galaxies. The models assume that the expansion and internal pressure of individual HII regions is driven by the net input of mechanical energy from the central cluster, be it through winds or supernova events. This eliminates the ionization parameter as a free variable, replacing it with a parameter that depends on the ratio of the cluster mass to the pressure in the surrounding interstellar medium. These models explain why HII regions with low abundances have high excitation and demonstrate that at least part of the warm ionized medium is the result of overlapping faint, old, large, and low-pressure HII regions. We present line ratios (at both optical and IR wavelengths) that provide reliable abundance diagnostics for both single HII regions or for integrated galaxy spectra, and we find a number that can be used to estimate the mean age of the cluster stars exciting individual HII regions.