Leptomeningeal carcinomatosis occurs in approximately 5% of patients with cancer. This disorder is being diagnosed with increasing frequency as patients live longer and as neuro-imaging studies improve. The most common cancers to involve the leptomeninges are breast cancer, lung cancer, and melanomas. Tumour cells reach the leptominges by hematogenous spread or by direct extension from pre-existing lesions and are then disseminated throughout the neuroaxis by the flow of the cerebrospinal fluid. Patients present with signs and symptoms from injury to nerves that traverse the subarachnoid space, direct tumour invasion into the brain or spinal cord, alterations in blood supply to the nervous system, obstruction of normal cerebrospinal fluid (CSF) flow pathways, or general interference with brain function. The diagnosis is most commonly made by lumbar puncture although the CSF cytology is persistently negative in about 10% of patients with leptomeningeal carcinomatosis. Radiologic studies may reveal subarachnoid masses, diffuse contrast enhancement of the meninges, or hydrocephalus without a mass lesion. Without treatment, the median survival of patients with this disorder is 4-6 weeks and death occurs from progressive neurologic dysfunction. Early diagnosis and therapy is critical to preserving neurologic function. Radiation therapy to symptomatic sites and disease visible on neuroimaging studies and intrathecal chemotherapy increases the median survival to 3-6 months. The major favorable prognostic factors include excellent performance status, absence of serious fixed neurologic deficits, normal CSF flow scans, and absent or responsive systemic tumour. Aggressive therapy for this disorder is often accompanied by a necrotizing leukoencephalopathy which becomes symptomatic months after treatment with radiation and intrathecal methotrexate. As currently available therapies are toxic and provide limited benefits, novel approaches are being studied. Further information on the mechanisms of neurotoxicity from antineoplastic agents is critical to providing better outcomes for this increasing common complication of cancer.