Today sewage is the largest pollutant on a global scale, as particularly in developing countries, only a small fraction of the sewage produced is treated. Sewage treatment by conventional means, including secondary aerobic biological treatment, is efficient. But this efficiency is at the price of high capital and running cost and technology requirement. Alternatively, anaerobic treatment has been proven to be an admirable process and considered as the core of sustainable waste management. This lesson covers some of the fundamental principles of the anaerobic processes. Then the concept of High rate systems has been introduced and the advantages and drawbacks of applying those systems has been introduced. The application of the Upflow anaerobic sludge blanket (UASB) reactor in tropical and low and moderate temperature countries has been presented. The key issues and approaches of UASB reactor design including guidelines for the design of the physical components of the reactor are discussed, in addition to a design example.