Abnormal eye movements found in patients with central nervous system (CNS) diseases can sometimes be revealing about the underlying pathology. Although the oculomotor nuclei, their descending primary motor neurons, and the extraocular muscles they innervate ultimately mediate eye movements, higher centers (including premotor areas in the brain stem, midbrain, subcortical areas, and cerebral cortex) are also involved in the movements. The oculomotor nuclei and neurons, together with the higher motor centers, functionally comprise five oculomotor systems responsible for different types of eye movements. Of these, the smooth pursuit eye movement system and saccadic eye movement system are of interest, for they seem to be abnormal in CNS disorders where disease in higher subcortical and cortical centers is implicated. Moreover, both smooth pursuit and saccadic eye movements have distinct and easily measured properties. Disorders of smooth pursuit eye movements (SPEM) have been found in schizophrenia, CNS illnesses affecting the oculomotor nuclei (e.g., brainstem infarction), diffuse brain diseases (such as disorders of the Alzheimer's type), Huntington's disease, and Parkinson's disease.