  In this paper we discuss the calculation of chemical abundances in planetary
nebulae and H II regions through ionization correction factors (ICFs). We
review the first ICFs proposed in the literature based on ionization potential
similarities and we present the most recent ICFs derived from large sample of
photoionization models. We also discuss some of the considerations that have to
be kept in mind when using ICFs to compute the chemical composition of ionized
nebulae.
