A mechanistic model was developed to assist with design of constructed wetlands intended to treat industrial wastewater in the Arabian Gulf region. The model is deterministic in nature and thus allows explicit evaluation of the fate and transport of wastewater constituents throughout constructed wetlands to assess their efficacy in treatment. Although the model was developed for the purpose of evaluating treatment wetlands in the hot and arid climate of the Middle East, the model is generalized in order to allow users to simulate a wide range of climates, any type of wetland, with any configuration, and any selection of material and plants. The model simulates the majority of physical, chemical, and biological processes that could occur in wetlands that are responsible for the removal of constituents. The model is designed to be flexible enough to allow the simulation of any number pre-selected constituents or other constituents of the user’s choice made based on the quality of influent. The model relies on a large body of existing literature to formulate each process. The model development discussed in this report and the resulting model, CWM, is considered “in progress”. During the present stage, the model framework and algorithms have been developed and made functional. Subsequently, model formulations will need to be validated, model limitations analyzed and addressed, and model refinements made, before and during use in full-scale operations and design. As such, it is recognized that the present stage of model development is “experimental”, and future refinements of the model are anticipated. Model results at this time consist of evaluation of key model features and how they perform in Middle Eastern climates.