The bulk of research, development, and application of constructed wetland systems (CWSs) have taken place in the past 20 years. There are now thousands of these systems being used in Europe. Many are used for individual houses or small groups of houses, but the most common use is for treating the domestic sewage from villages to achieve secondary treatment. There are also larger systems for tertiary and storm sewage overflow treatment. The processes have spread out from domestic wastewater treatment into many other areas, such as mine water treatment and industrial effluent treatment. Development has been more rapid than is usual for wastewater treatment processes, and the processes are now regarded as conventional. One of the reasons is that the processes are seen as both "green" and cost-effective. Another reason for the rapid development of CWSs is that there has been very close national and international cooperation. This chapter describes the stepwise development of treatment applications from the simple horizontal flow beds for biochemical oxygen demand measured over 5 days and total suspended solids removal in the mid-1980s, moving on to vertical flow beds capable of nitrification in the early 1990s. The development of hybrid systems capable of the previous treatment applications plus denitrification in the late 1990s will be described together with the main advances.