Echo is one of the major issues affecting quality of service in Voice over Internet Protocol (VoIP) implementations. While echo is present in conventional circuit-switched telephone networks, echo becomes a more complex issue in VoIP networks because of the need to compensate for the greater round trip delay inherent in VoIP networks. This paper presents an overview on the need to provide echo cancellation in voice networks that interconnect traditional telephone networks (PSTN) and IP networks such as the Internet. Information includes what is echo and what causes it; echo cancellation, echo management in the PSTN, echo in tandem networks, and echo as it relates to VoIP.