Nitrogen exhibits an intriguing combination of highly volatile behavior (particularly as N2), appreciable reactivity, and surprising compatibility in the deep Earth. Nitrogen is incorporated into the biosphere and then, through diagenesis and low-grade metamorphism, is conveyed into the lithosphere and the deeper Earth. The investigation of N behavior in the biosphere, hydrosphere, and atmosphere has led to many important discoveries regarding biogeochemical pathways, including in areas such as trophic interactions and anthropogenic impacts on terrestrial and marine environments (e.g. nutrient pollution, eutrophication). Nitrogen can act as an excellent tracer of the transfer of sedimentary/organic materials into and within deep-Earth reservoirs and shows great potential as a tracer of life on early Earth and elsewhere in the Solar System.