This technical report summarizes T1's current views on the principles of the operation of echo cancellers and their applications. It includes a historical background for the use of echo control devices in the Public Switched Telecommunication Network (PSTN). It also reviews application rules and operational constraints for pubic/private network transmission planning. The report discusses the relationship among planners of PSTNs, modem manufacturers, private network planners, and end users regarding the control of echo (from sources inside and outside the PSTN) and the associated terminal considerations.