HII regions are usually treated theoretically as containing zero-age stars of given metallicity, and choosing a radius to give the right ionization parameter. However, the stars exciting real HII regions evolve rapidly with time, while their stellar winds inflate mass-loss bubbles around them. I will show how a self-consistent theory of such realistic time-dependent HII regions has been constructed, and how the dimensionality of this problem can be reduced by one, thus making it much more tractable. With this theory, one can define HII region isochrones for individual HII regions, allowing the age and metallicity to be independently derived from line excitation diagrams. For starburst galaxies, we can derive integrated spectra, which are directly comparable with observation, and we can compute IR continuum spectra which can be compared with Spitzer IRS spectra to give star formation rates, interstellar gas pressures and other physical parameters of the starburst.