Since nitrogen is a key element in the biosphere, it was natural that as techniques became generally available for work with isotopes, those of nitrogen would be utilized. Approximately, a decade passed before nitrogen isotope techniques became fully established in marine research, but most studies have employed the stable isotope, rather than the radioactive isotopes. 13N is used currently in a small number of experiments but its short half-life restricts its use. 15N can be employed in two ways; first, as a tracer in which reactions involving nitrogen are followed after addition of quantities of 15N in various chemical forms. The second, and the subject of this review, is the investigation of the variations in the concentration of 15N in natural materials. The study of the natural variations in 15N can provide information in a wide variety of investigations, including biogeochemical, physical, physiological, and biochemical investigations. However, despite the wide variety of applications, variations in 15N are invariably employed as a natural tracer— that is, to track the sources and sinks of nitrogen under natural conditions. This chapter covers aspects of the natural variations in 15N in a wide range of subject areas, for example, dissolved gases in the deep oceans to the dietary relations of laboratory grown animals. It attempts to draw together many of the studies carried out in the marine environment.