Submitted to: IEEE Communications Magazine Special Issue on Internet Telephony

N.B. Sections in italic denote additions w.r.t. submitted version. We discuss the technical viability of transporting real-time voice over IP networks when the output rate of the speech encoder is adjusted to match the characteristics of the network: speech is coded at low bit rates when the network is congested; at higher bit rates, otherwise. First, issues affecting perceptual QoS are analyzed. The rationale for Adaptive Voice over IP (AVoIP) is explained and possible architectures described. The problem of estimating the state of the network is then discussed. A rate control algorithm is introduced. Finally, measurements of AVoIP performance over the Internet are presented. Adaptive voice over IP solutions allow more efficient use of network resources, offer the operator greater flexibility and pave the way for new services.