This article outlines the isotope systematics of the major processes in the ocean nitrogen (N) cycle, their impact on the isotopic compositions of the major N reservoirs in the ocean, and how the N isotopes in the modern ocean and sedimentary record 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 relatively enriched in 15N. The description of models and of observed N fluxes and pools in the ocean is dominated by two general parameters: (1) the amplitude of the isotope discrimination in a given N conversion, and (2) the degree to which that conversion consumes its substrate. © 2009 Copyright © 2009 Elsevier Ltd. All rights reserved.