In many countries around the world, it is necessary to upgrade municipal wastewater treatment plants (WWTPs) to meet the more stringent discharge limits of nutrients. However, with the decreased total dissolved nitrogen (TDN) in the effluent, the proportion of dissolved organic nitrogen (DON) in TDN is increasing, which potentially contributes to the synthesis of more nitrogenous disinfection byproducts (N-DBP) and the eutrophication of receiving water bodies. As a result, studies on WWTP DON have been increasing in recent years, and this article systematically reviews its characteristics, transformation and ecological consequences. At present, up to 70% of the influent DON can be removed by coagulation, precipitation, disinfection and other combined physical and chemical processes. However, metabolic activities in the biological treatment units generate new DON, including small molecular amino acids and polyamines, which have a relatively high algal bioavailability. We propose a more comprehensive ASM3-DON model, involving six major processes associated with DON biotransformation, including endogenous respiration, cell growth, and reuse of soluble microbial products. The model can be used to predict wastewater effluent DON concentration with increased accuracy. Future research on wastewater DON should focus on developing more rapid and accurate quantification methods, unraveling its formation and transformation mechanisms, as well as developing more efficient removal techniques to reduce effluent DON.