We study the physical structure of H II regions, including Planetary Nebulae (PNe), through detailed analysis of the ionization structure and spectra of iron in relevant ionization stages. This results from our work in the Ohio State group on: (i) calculation of new and improved atomic data for Fe I-VI (photoionization cross sections, recombination rates, collision strengths, and transition probabilities); (ii) detailed study of ionization, recombination, and excitation mechanisms and radiative transfer effects under nebular conditions; (iii) study of the physical structure and kinematics of nebulae and their ionization fronts. This includes further examination of high density regions deduced from the spectroscopic analysis of optical [Fe II], [Ni II], and [O I] emission (Bautista etal 1996; and references therein). The study also addresses the issue of chemical abundances in fully ionized and partially ionized zones of nebulae.