A symposium at the FASEB meeting in Las Vegas in May 1988 will be devoted to the role of cofactors (vitamins, coenzymes, prosthetic groups) in and as posttranslational protein modifications; the symposium is part of a thematic focus on metabolic regulation. In planning the symposium, we decided to consider metabolic regulation in its broadest context, which should include both the short‐term activity modulations in the life of contemporary organisms and the adaptations of special molecular strategies over evolutionary time. We further decided to focus the symposium context on the involvement of cofactors both as catalytic partipicantsinandassubstrates or end products of posttranslational modifications. As a preview of the actual symposium, the present discussion is an attempt to enumerate cases of cofactor involvement in these different categories:1) essential nutrients as participants in posttranslational modifications;2) cofactors as donor substrates in reversible, regulatory modifications; and3) cofactor incorporation or generation as covalent constituents of proteins. The actual symposium topics are taken from category1: vitamin C and protein hydroxylation (K. I. Karivikkio) and vitamin K and protein carboxylation (J. W. Suttie) and category3:biotinylation (H. G. Wood), phycobiliproteins (A. Glazer), and pyruvoyl enzymes (W. Dowhan).— Rucker, R. B.; Wold, F. Cofactors in and as posttranslational protein modifications.FASEB J.2: 2252‐2261; 1988.