The cerebellum is the most commonly affected part of the brain in autistic spectrum disorders (ASDs). The histopathological changes strongly indicate selected damage to particular cell groups and lobules of the cerebellum rather than diffuse injury. A number of studies have shown injury and abnormal development of the vermis of the cerebellum, with a predominance of neuronal loss among Purkinje cells and granule cells. In addition, one see abnormal pathway development indicating intrauterine damage or damage occurring during the early postnatal period. Several studies have shown abnormalities of glutamate receptors (GluRs) of various kinds, including metabotropic GluRs (mGluRs). In this chapter, I review the histopathologic findings within the ASD cerebellum and demonstrate evidence for immunoexcitotoxicity affecting cerebellar neurodevelopment as well as evidence for early neurodegeneration. Newer studies have shown that the cerebellum may have significant cognitive and higher cortical functions, either by way of its connections to prefrontal-limbic areas or more indirect pathways.