Models of H II regions provide ionization corrections for abundance determinations and estimates of the ionizing star temperature Tasterisk and of the nebular ionization parameter U. In the absence of observed electron temperatures, models may be used to determine abundances; and a systematic procedure is described here. Accurate observations of forbidden O I, forbidden Ne III, and forbidden S III are needed to carry out this procedure. For giant extragalactic H II regions there may be a tendency for both Tasterisk and U to decrease with increasing O/H. The small volume-filling-factors and characteristic values of U in these objects may result from the dynamical effects of stellar winds.