Constructed wetlands (CWs) provide a natural way for simple, inexpensive, and robust wastewater treatment. Detailed understanding of CW functioning is difficult, because a large number of physical, chemical, and biological processes occur in parallel and influence each other. For this reason, CWs have long been seen as “black boxes” where wastewater enters and treated water leaves the system. Numerical models describing the biochemical transformation and degradation processes in CWs are promising tools to better understand CW functioning. The first part of this paper reviews published mechanistic models for CWs. Horizontal flow systems can be simulated when only water flow saturated conditions are considered; six models have been reviewed whereby a series or network of completely stirred tank reactors is most frequently used to describe the hydraulics. For modeling vertical flow CWs with intermittent loading, transient variably saturated flow models are required. Due to the intermittent loading, these systems are highly dynamic, adding to the complexity of the overall system. Five models of different complexity have been reviewed; three use the Richards equation to describe variably saturated flow, whereas the two others use simplified approaches. In the second part of the paper, the multicomponent reactive transport module CW2D is demonstrated. Simulation results for CWs treating domestic wastewater, combined sewer overflow, and surface water are presented. In general, a good match between simulation results and measured data could be achieved if the hydraulic behavior of the system could be described well. Based on the experience from these examples, the need for further model development is determined.