The anaerobic treatment process is increasingly recognized as the core method of an advanced technology for environmental protection and resource preservation and it represents, combined with other proper methods, a sustainable and appropriate wastewater treatment system for developing countries. Anaerobic treatment of sewage is increasingly attracting the attention of sanitary engineers and decision makers. It is being used successfully in tropical countries and there are some encouraging results from subtropical and temperate regions. In this review paper the main characteristics of anaerobic sewage treatment al-e summarized with special emphasis on the upflow anaerobic sludge blanket (UASB) reactor: The application of the UASB process to the direct treatment of sewage is reviewed, with examples fr-om Europe, Asia and the Americas. The UASB reactor appears today as a robust technology and is by far the most widely used high-rate anaerobic process for sewage treatment.