Constructed wetlands are designed to treat nitrogen inputs in storm- and wastewater runoff generated in urban and industrial catchments. These systems utilise a number of natural processes; such as assimilation and denitrification, that store and remove nitrogen, respectively, preventing further transport downstream. Several studies have assessed the effectiveness of nitrogen removal in constructed wetlands in the initial design phase and shortly after construction. However, aside from monitoring inlet and outlet concentrations of nitrogen there is very little information gathered on the ongoing function of the system. We propose that natural abundance stable isotopes of NO3-, NH4+, plant-N, periphyton-N and sediment-N can provide useful qualitative information on wetland nitrogen pathways, allowing for more informed management of these systems. Nitrogen fractionates in a predictable fashion when it is processed, as such, we can use these levels of fractionation (15ε) to identify the key processes, such as denitrification (15ε = ~2 ‰ δ15N-NO3-) and assimilation (15ε = 5-15 ‰ δ15N-NO3-), occurring in the system. These methods were employed in three constructed urban wetlands in Melbourne and Perth, Australia using transect sampling before and after a rainfall event. Initial results indicate that δ15N-NO3- could be a useful tool in assessing wetland function. The results of more detailed studies will also be discussed.