Evapotranspiration (ET) is a method of onsite wastewater treatment and disposal that is an alternative to conventional soil absorption systems, particularly for sites where protecting surface water and ground water is essential or where soil infiltration is not possible. One of the most important aspects of ET systems is their ability to evapotranspire all of the sewage discharged into the systems and the rain falling onto the systems. On an annual basis the ET should equal the amount of wastewater discharged into the system plus the amount of precipitation falling onto the system. Part of the nutrients can be recycled via the plant biomass, and the harvested biomass can serve as a source of bio-energy. In Denmark more than 500 ET systems planted with willows are in operation. The systems generally consist of a 1.5 m deep high-density polyethylene-lined basin filled with soil and planted with clones of willow (Salix viminalis L.). The surface area of the systems depends on the amount and quality of the sewage to be treated and the local annual rainfall. A single household in Denmark typically requires between 120 and 300 m2. The annual precipitation at the site of construction is an important dimensioning parameter. Settled sewage is dispersed underground into the bed under pressure. The stems of the willows are harvested on a regular basis to stimulate the growth of the willows and to remove some nutrients and heavy metals. In this paper, the theory behind the operation of willow based ET systems, their design, construction and management as well as operational experience are described.