The relative amounts of the two stable isotopes of Nitrogen (N), 15N and 14N, vary predictably in soils and plant tissues of forests and other non-cultivated ecosystems. Slight fractionations, or discriminations against the heavier N isotope, that can occur as N cycles through vegetation, soils and microbial biomass act over the long term to deplete vegetation and litter of 15N and to enrich humus in 15N. In this chapter, the authors show that 15N natural abundances can be used to make inferences about (1) rates and patterns of N cycling, (2) the relative importance of N inputs versus soil processes in supplying N for plant uptake, (3) retention of N by forests and (4) forest ecosystem health. The authors also discuss uses of 15N-labelling ecosystem-scale experiments on whole forests. Such experiments offer possibilities for checking and improving estimates of N fluxes within and N losses from forests.