  This chapter presents a review on the latest advances in the computation of
physical conditions and chemical abundances of elements present in photoionized
gas H II regions and planetary nebulae). The arrival of highly sensitive
spectrographs attached to large telescopes and the development of more
sophisticated and detailed atomic data calculations and ionization correction
factors have helped to raise the number of ionic species studied in
photoionized nebulae in the last years, as well as to reduce the uncertainties
in the computed abundances. Special attention will be given to the detection of
very faint lines such as heavy-element recombination lines of C, N and O in H
II regions and planetary nebulae, and collisionally excited lines of
neutron-capture elements (Z >30) in planetary nebulae.
