Water is the most plentiful liquid on earth. It is a requirement for all life, whether it is used in metabolic processes, as a solvent of minerals, or in the disposal of waste. Indeed, it is difficult to name a natural phenomenon in which water, in one form or another, has not had an essential role. Nature has provided for the conservation of this abundant and valuable liquid by endowing it with a considerable capacity to rid itself of foreign substances. This process, known as self-purification, is an inherent system for water. Man's only problem with the system is that the water does not always clean itself quickly and completely enough for the next intended reuse. Almost all of the processes used by man to clean the water are accelerated natural-quality recovery mechanisms. The water may be moving in streams, or be relatively static as in lakes or the oceans, but the natural processes of purification are proceeding toward completion. Advantage is taken of this intrinsic ability of water to purify itself by using it as the final polishing treatment for all municipal and industrial waste discharges. Streams contaminated by natural, human, or industrial wastes are rendered stable by self-purification. Eventually, this water becomes innocuous and is again reused by man or nature. This purification is accomplished by combinations of the physical, chemical, and biological forces present in the water environment. The extent to which each force operates is dependent upon the contaminating substance and the particular water environment in which it is present. Physical forces separate suspended solids; chemical forces cause reactions which neutralize the nocuous wastes; and biological forces stabilize the water by using the contaminating substances as nutrient sources and by enchancing both physical and chemical phenomena. Generally, the biological forces are the most active and the most important in the self-purification of a stream.