Using a basic biogeochemical approach, constructed wetland treatment systems (CWTSs) can be designed to renovate contaminated waters for beneficial reuse. The purpose of this article is to present the fundamental design strategy for CWTSs for a variety of contaminated waters. In designing a CWTS, the contaminants of concern are identified in the water to be treated, and effective biogeochemical pathways by which the targeted constituents can be transferred or transformed are determined. Specific transfer processes in wetland cells of a CWTS include sorption, volatilization, precipitation (and settling), and bioconcentration. Transformation processes in the wetland cells include photolysis, hydrolysis, speciation and ionization, oxidation, reduction, and biotransformation. Physical models (pilot-scale CWTSs) are built according to the process-based design, and their performance is measured in terms of rate and extent of removal of targeted constituents as well as functional parameters indicating readiness to perform. Demonstration-scale systems may be used to provide additional site-specific data. Full-scale CWTSs are designed for site conditions, and performance is monitored as part of the operation and maintenance to ensure treatment. A variety of contaminated waters can be treated effectively and efficiently using well-designed CWTSs.