To many people Constructed Wetland is nothing but a pond filled with aggregates from which plants grow. It does not portray a picture of a reactor in which complex chemical and/or biological processes take place with the end result of treated wastewater that can be discharged in the environment with little or no adverse impact. In fact a constructed wetland (CW) is an artificial wetland created for the purpose of treating anthropogenic discharge such as municipal or industrial wastewater, or storm water runoff. They are engineered systems that use natural functions of vegetation, soil, and organisms to treat different water streams. To demonstrate this fact this book illustrates the immense chemical / biochemical engineering theories and principles in the design and operation of these systems.