This report summarises the information found in scientific literature regarding the mechanisms and processes that enable constructed wetlands to remove heavy metals and pesticides from waste water. It examines what factors have an influence on the effectiveness of constructed wetlands to treat waste water containing such pollutants and shows the impact on the design and operation of constructed wetlands. It focuses on free surface flow wetlands and aquatic treatment systems because these types are found to be most able to treat the large volumes of water that are required to fill and maintain water levels of fish ponds as operated in Egypt. CDI report no. CDI-13-019.