The treatment of industrial and domestic wastewaters by passage through beds containing plants of the common reed (Phragmites australis), reedmace (Typha latifolia), or other species, has been widely practised in recent years, with varying degrees of success. Although many workers have demonstrated good removal of organic components of effluents and of suspended solids, poor removal of ammoniacal nitrogen is a common finding. This has often been shown to limit the value of reed beds for treatment of raw landfill leachates. Engineered wetlands do however, have considerable capability for secondary polishing of leachates that have been pretreated in aerobic biological plants and for older leachates, this technology having been widely used in the UK. This paper will deal in general and in detail with the design of engineered wetland systems, both for the treatment of weaker and more-diluted leachates from older landfill sites, and also for the polishing of effluents from the pretreatment of much stronger leachates in aerobic biological treatment plants.