This chapter deals with neuropsychological disturbances in patients with bilateral cerebellar damage (BCD), i.e., epileptic patients chronically receiving phenytoin, patients with olivopontocerebellar atrophy (OPCA), and Friedreich's ataxia (FA) versus those with unilateral cerebellar damage (UCD), i.e., patients with cerebellar strokes. BCD patients showed: (i) impaired executive functions in planning and programming of daily activities, elaborating and using structures, and difficulty in abstract thinking, functions that are related to cerebello-frontal loops; (ii) deficits in visuospatial organization for a concrete task and deficient visual-spatial working memory, functions related to cerebello-parietal loops; (iii) lower general intellectual abilities than controls (especially those with OPCA); (iv) difficulties with memory retrieval, diminished global memory quotient, and reduced spatial working memory ability; and (v) slower speed of information processing, as measured by simple and multiple choice reaction time (RT). In UCD patients, neuropsychological and neurobehavioral abilities were deficient for 2-5 months; after this time period, their performances returned to normal. In both BCD and UCD patients, single photon emission computed tomography (SPECT) studies showed different degrees of "reverse" cerebellar-->basal ganglia-->frontoparietal diaschisis which may underlie permanent or transitory neuropsychological deficits. The relationships among neuropsychological findings, SPECT studies, and chemical neuroanatomy are discussed.