This article outlines the isotope systematics of processes in the ocean nitrogen (N) cycle and their impact on the isotopic compositions of the major N forms, with an eye toward how the N isotopes may record current N fluxes and past changes. The dominant dynamic is of kinetic isotope discrimination, in which the light isotope of N (14N) is preferentially converted from substrate to product, leaving the substrate enriched in 15N. The strength of the isotopic signal increases with the amplitude of the isotope discrimination in a given N conversion, and it is also affected by the degree to which that conversion consumes its substrate, with a higher degree of consumption strengthening the signal in the substrate but weakening it in the product. © 2019 Elsevier Ltd.