In order to find a reliable, simple and cost-effective small wastewater treatment system, an experimental research programme on duckweed-based treatment systems was carried out. In spite of the profitable characteristics of duckweed (high productivity, high protein content, wide geographic distribution, control of negative impacts from conventional wastewater treatment ponds), the results obtained confirm that their extensive use in Italy seems difficult because of the high requirement of land area and the ceasing of growth in winter months (at least in Northern Italy). In temperate climates, a reasonable use of duckweed looks to be the production of good quality secondary effluents (BOD and SS removal) from small communities, especially in seasonal (summer) wastewater treatment plants. Another use is algae removal from facultative lagoon effluents. Nitrogen removal can only be reasonably obtained in duckweed-covered ponds with supplemental aeration. Because of several constraints due to the markets and to environmental regulations, harvested duckweed has to be disposed of as sewage sludge (e.g. compost or biogas production).