Major progress has been made over the last years in our understanding of the mechanisms underlying immune privilege and immune surveillance of the central nervous system (CNS). Once considered a passive process relying only on physical barriers, immune privilege is now viewed as a more complex phenomenon, which involves active regulation of immune reactivity by the CNS microenvironment. Evidence has also emerged that the immune system continuously and effectively patrols the CNS and that dysregulated immune responses against CNS-associated (exogenous or self) antigens are involved in the pathogenesis of various neurological diseases. In this article we shall briefly review current knowledge of how the immune response is regulated locally in the CNS and which cell types and molecular mechanisms are involved in shaping intracerebral immune responses.