Despite significant advances in the treatment of systemic cancers, progress in the treatment of primary brain tumors has been quite modest. In addition, an increasing proportion of patients with systemic cancers are presenting with brain-only metastases. These observations highlight the critical role that the blood-brain barrier plays in preventing antineoplastic therapies from reaching the central nervous system in therapeutic concentrations. This review describes the anatomy of the blood-brain barrier and currently available methods to quantify the entry of therapeutic compounds into the brain. It also summarizes data from a variety of approaches designed to improve drug delivery to the central nervous system. These include: 1) directly placing drugs inside the blood-brain barrier (polymeric implants, convection-enhanced delivery, and intraventricular administration), 2) modifying systemic chemotherapy (by using high-dose methotrexate and intra-arterial drug administration), 3) temporary disruption of the blood-brain barrier (via use of intra-arterial mannitol, focused ultrasound, or pharmacologic agents), and 4) designing drugs that can pass through the blood-brain barrier. Given that lymphocytes readily traverse the blood-brain barrier, immunotherapy represents a novel approach to cancer therapy that is of particular interest to practitioners in the field of neuro-oncology. The efficacy of vaccines and immune checkpoint inhibitors is currently being actively investigated in patients with primary and metastatic brain tumors, as well as leptomeningeal carcinomatosis. The challenge of delivering effective antineoplastic therapies to the central nervous system remains a primary obstacle to improving outcomes in patients with primary and metastatic brain tumors.