The possibilities of in-sewer treatments of wastewaters are broadly covered, including discussions of biological, chemical, and physical methods. The best possibilities are in the direction of biological oxidation, leading to partial reduction of the BOD, or extending to the equivalent of standard complete treatment. An appreciable reduction of BOD occurs normally in sewers. The amount of this oxidation can be estimated by equations developed in this and other closely related research. The requirements for a high degree of treatment are an adequate oxygen supply, an oxidizing culture, and time. The culture may be suspended in the form of an activated sludge, or attached to solid surfaces. The methods for supplying atmospheric or industrial oxygen are explored. The prospects for a useful degree of in-sewer treatment, especially in force mains, are very good. Under certain limited conditions the sewer can function as an efficient flocculation device in conjunction with chemical treatment. (Color illustrations reproduced in black and white)