The analysis of giant HII regions using photoionization models

After briefly reviewing the impact of various parameters on photoionization models, we illustrate the importance of the nebular geometry by two examples: we first compare the results from models with thin shell and full shell geometry, and we study the influence of a diffuse ionized medium in the integrated spectra of galaxies at large redshift. Finally, we show how the calibration of the strong line method for deriving oxygen abundances depends on the assumed average properties of the ionizing star cluster and on the density distribution of the nebular gas.