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I porting that schizophrenia patients showed an abnormality in following a smoothly moving target with their eyes. Subsequent reports showed that the same abnormality occurred in about 40% of the firstdegree relatives of schizophrenia patients; that the phenomenon was not a function of neuroleptic drug treatment, measurement artifact, population characteristics, or inattention; and that performance remained stable over time, even when measurements were separated by several years (Iacono & Lykken, 1981; Levy, Lipton, Holzman, & Davis, 1983). Several replications of the essential findings quickly appeared, the earliest being those of Iacono and Lykken (1979) and Shagass, Amadeo, and Overton (1974). Reviews, appearing periodically during the intervening 25 years, have recounted the many replications of the basic phenomenon, the most recent by Levy, Holzman, Matthysse, and Mendell (1993). In this chapter, we explore the nature of this abnormality and trace one of its components to a disorder in speed discrimination. After a brief description of the attributes of smooth pursuit eye move-