Numerous technologies are available for the treatment of wastewater. Many systems have been constructed and successfully operated ranging from simple on-site systems to sophisticated large-scale systems with computer operational control. In evaluating the technologies for application in a particular situation many factors have to be considered. These include capital cost, availability of fund, financing arrangement, cost-recovery possibility (affordability by the users), operating and maintenance, and the need for training in operation and maintenance. There is also the wider consideration of planning to set land for the sewerage pipes, pumping stations and the treatment plant, integration of wastewater services with stormwater drainage and with solid waste disposal, community involvement and the local government and non-government processes to implement the wastewater collection and treatment project. Though these factors have to be thoroughly considered to ensure long-term viability and sustainability of a wastewater management system, an understanding of the principles of wastewater treatment is essential in enabling a proper evaluation of treatment technologies for possible application in a local situation. We need to be able to answer the question of whether a particular technology will work or indeed appropriate considering the prevailing economic, social, environmental and institutional factors mentioned above. Will a high-technology high-cost system be the answer, or will a low-cost on-site system be adequate, or will a community-scale system be the most appropriate given the set of local factors? Understanding how these technologies work will go along way towards answering the question. The understanding will also enable us to assess the adequacy of existing local technology (one that has been in used locally over many years), how to improve the existing technology, or how to adapt one of the available technologies to better fit the local condition. The purpose of this paper is to develop principles for understanding wastewater treatment by first of all examining the natural processes taking place in nature that help to purify wastes. These principles are then applied to the examination of simple wastewater treatment systems that closely mimic nature. The natural physical, biological and chemical processes are then related to engineered systems which are more complex and where separate units may be constructed to carry out the processes. In this paper emphasis is placed on small-scale on-site engineered systems.