This work reviews applications of stable isotope analysis to the studies of transport and transformation of N species in groundwater under agricultural areas. It summarizes evidence regarding factors affecting the isotopic composition of NO₃⁻, NH₄⁺ and N₂O in subsurface, and discusses the use of ¹¹B, ¹⁸O, ¹³C, ³⁴S, ⁸⁷Sr/⁸⁶Sr isotopes to support the analysis of δ¹⁵N values. The isotopic composition of NO₃⁻, NH₄⁺ and N₂O varies depending on their sources and dynamics of N cycle processes. The reported δ¹⁵N-NO₃⁻ values for sources of NO₃⁻ are: soil organic N – + 3‰–+8‰, mineral fertilizers – − 8‰–+7‰; manure/household waste – + 5‰ to + 35‰. For NH₄⁺ sources, the isotopic signature ranges are: organic matter – + 2.4–+4.1‰, rainwater – − 13.4–+2.3‰, mineral fertilizers – − 7.4–+5.1‰, household waste – + 5–+9‰; animal manure – + 8–+11‰. For N₂O, isotopic composition depends on isotopic signatures of substrate pools and reaction rates. δ¹⁵N values of NO₃⁻ are influenced by fractionation effects occurring during denitrification (ɛ = 5–40‰), nitrification (ɛ = 5–35‰) and DNRA (ɛ not reported). The isotopic signature of NH₄⁺ is also affected by nitrification and DNRA as well as mineralization (ɛ = 1‰), sorption (ɛ = 1–8‰), anammox (ɛ = 4.3–7.4‰) and volatilization (ɛ = 25‰). As for the N₂O, production of N₂O leads to its depletion in ¹⁵N, whereas consumption – to enrichment in ¹⁵N. The magnitude of fractionation effects occurring during the considered processes depends on temperature, pH, DO, C/NO₃⁻ ratio, size of the substrate pool, availability of electron donors, water content in subsoil, residence time, land use, hydrogeology. While previous studies have accumulated rich data on isotopic composition of NO₃⁻ in groundwater, evidence remains scarce in the cases of NH₄⁺ and N₂O. Further research is required to consider variability of δ¹⁵N-NH₄⁺ and δ¹⁵N-N₂O in groundwater across agricultural ecosystems.