The classic paraneoplastic neurological syndromes include Lambert-Eaton myasthenic syndrome, limbic encephalitis, sensory neuronopathy, intestinal pseudo-obstruction, subacute cerebellar degeneration, encephalomyelitis, and dermatomyositis. Approximately ten onconeural antibodies that recognize cancer and the nervous system have been described in paraneoplastic neurological syndromes. These antibodies appear to be important diagnostic tools, even though they may not always be present. Deciding whether a given neurological picture is definitely or possibly paraneoplastic depends on the clinical syndrome, any association with onconeural antibodies, and the time elapsed between onset of neurological symptoms and the discovery of the cancer. Diagnosis of a classic paraneoplastic neurological syndrome or the discovery of onconeural antibodies mandates an active and persistent search for cancer, using new techniques such as fluorodeoxyglucose positron emission tomography. In patients with one of these syndromes, the best treatment of the neurological disease is often the diagnosis and early treatment of the cancer.