We present theoretical evolutionary models for ionizing star clusters and their associated giant HII regions up to an age of 5.4 Myr and metallicities between 1/20 and 2.5 times solar. These young clusters can provide the high energy photons needed to keep the regions ionized. We discuss in some detail the Wolf-Rayet and red supergiant phases, sometimes present in the early stages of the evolution of a star cluster. The spectral features characteristic of these stars, when detected and measured on the spectra of giant extragalactic HII regions (GEHR), can be used to constrain the age of the ionizing stellar population. The emergent ionizing continua of the clusters are used as input for the photoionization code CLOUDY to obtain the corresponding emission line spectra in the optical and infrared ranges. The effect of the metallicity is taken into account in the stellar evolution and atmosphere models as well as in the nebular gas producing a consistent set of models. These models can provide estimates of the mass, age and metallicity of the dominant stellar population ionizing a given GEHR solely from the analysis of its optical emission line spectrum.
