Constructed wetlands (CWs) subjected to variable loads present a series of challenges for designers and researchers. Classical design approaches (e.g. rule of thumbs or first-order kinetic model) are not suited to properly estimate the removal efficiencies of CW under varying flows. The internal removal processes of CWs are expected to be influenced by the variation of influent pollutant concentrations and hydraulic loads for particular CW applications (e.g., stormwater or combined sewer overflow treatment). A powerful tool to properly study and design CW under varying flows is given by mathematical model. Either for design or research purposes, mathematical models have been developed to simulate CW subjected to varying flow and are revised in this chapter. Models used to simulate the hydraulic behaviour as well as the treatment performances of variable flow CWs are reviewed. Moreover, future perspectives of mathematical models in this field are analysed in terms of design support tools, process-based model for design purposes, and limitation for a wider application.