This paper reviews the use of stable nitrogen isotopes (δ¹⁵N) to delineate the influence of sewage nitrogen (N) in coastal ecosystems, drawing extensively on the case of Himmerfjärden, a Baltic Sea bay that receives ¹⁵N-enriched tertiary treated sewage that is discharged mainly as dissolved inorganic N (DIN). Gradients of δ¹⁵N in macroalgae (Fucus vesiculosus) and surface sediments traced sewage-derived N to 24 km from the outfall but elevated δ¹⁵N values (> 7‰) indicated that the sewage influence was most pronounced within 10 km. Comparison of macroalgal δ¹⁵N values before and after enhanced tertiary treatment showed a decrease in the spatial impact of sewage N from about 24 km to 12 km from the outfall and a decrease to more marine δ¹⁵N values in more recent growth tissues. Sedimentary δ¹⁵N records showed that sewage has had a dominant influence on organic matter production in the bay with dramatic increases in sedimentary δ¹⁵N during the years of maximum sewage N loads. In cases where sewage N introduces a distinct isotopic signature into a system and where it has had a dominant influence on organic matter production, δ¹⁵N values in biota and sediments can be used to trace the spatial and temporal influence of sewage N in aquatic ecosystems.