This paper deals with the measurement of phase distortion or delay distortion and is particularly concerned with measurements on telephone circuits. For this purpose, use is made of a quantity defined as “envelope delay,” which is the first derivative of the phase shift with respect to frequency. Various methods for measuring this quantity and the principles on which they are based are discussed, the details of the measuring circuits being omitted and sources of further information referred to when possible. Data are included which give the measured envelope delay-frequency characteristics of several kinds of telephone circuits.