The paraneoplastic disorders of the nervous system are clinicopathologic syndromes that occur in association with systemic cancer. Dysfunction of any part of the nervous system or its effector organs may be produced by a paraneoplastic process. The temporal relationship between the onset of a paraneoplastic syndrome and the diagnosis of the associate cancer varies. Most of the paraneoplastic syndromes of the nervous system may appear months to years before the cancer is discovered. The pathogenesis of most of the paraneoplastic syndromes of the nervous system is unknown and specific, effective therapy is lacking. With rare exceptions, treatment of the associated cancer seldom alters the course of patients with these syndromes. However, in other paraneoplastic syndromes such as myasthenia gravis, myasthenic syndrome of Eaton-Lambert and polymyositis-dermatomyositis an autoimmune pathogenesis is established or strongly suspected.