The intra- and intermolecular chemistry of phenylnitrene (PhN), its singlet-triplet energy separation, and its electronic spectra are interpreted with the aid of ab initio molecular orbital theory. The key to understanding singlet PhN is the recognition that this species has an open-shell electronic structure, in contrast to the related species, phenylcarbene, which has a closed-shell electronic structure. The thermodynamics of nitrenes, benzazirines, dehydroazepines, aminyl radicals, and their hydrocarbon analogues are also discussed.