This chapter on biologic nitrogenous heterocycles deals exclusively with the coenzymatically active forms of vitamins. Seven of the eight B vitamins use heterocyclic chemistry, each one a separate ring system, to mediate chemical transformations at key nodal metabolic intersections. The heterocycles include single ring heterocycles: the thiazolium ion in thiamin-PP, the cationic pyridinium in NAD(P)⁺, and the pyridine aldehyde in pyridoxal-P. Bis-heterocyclic rings include biotin and folate coenzymes. Riboflavin offers a tricyclic isoalloxazine ring system and B12 a tetrapyrrolic framework. The ranges of chemistry enabled include carbanion, carbocation, and carbon radical intermediates, as well as hydride ion transfers.