Waste Stabilization Ponds (WSP) and Constructed Wetlands (CW) have proven to be effective alternatives for treating wastewater, and the construction of low energy-consuming ecosystems that use natural processes, in contrast to complex high-maintenance treatment systems, will hopefully lead to more ecologically-sustainable wastewater treatment in the future. CWs and WSPs also have the capability of meeting the demand for a high percentage removal of pathogenic organisms, compared to conventional technologies. CWs and WSPs combined, and joined with other technologies, may be important for even more improved performance of water cleaning systems. WSP’s and CW’s are now well-established methods for wastewater treatment in tropical climates. Their many advantages include: simplicity, low cost, low maintenance, low energy consumption, robustness, and sustainability. While WSPs are most commonly used for treating domestic wastewaters, they are also successfully used for treating industrial wastewater, including water that contains agro-industrial wastes. One of the potential advantages of using constructed wetlands is that they do not allow mosquitoes to breed (sub-surface flow wetland). The process of designing WSPs and wetlands, and predicting their performance, is improving rapidly as we gain more experience with these systems.