The subpulse drift phenomena—that is, periodic subpulse phase variations in the individual-pulse sequences—was discovered by Drake and Craft (1968) in the beginning of the pulsar era. Many observational programs have been carried out since then, and it has been shown a) that only a minority of pulsars exhibit the phenomenon, and b) that only some of these can be characterized by a definite drift rate (Huguenin, Taylor, and Troland 1970, Backer 1970, Taylor and Huguenin 1971).