Multidrug resistance (MDR), whereby tumor cells simultaneously possess intrinsic or acquired cross-resistance to diverse chemotherapeutic agents, hampers the effective treatment of cancer. Molecular investigations in MDR resulted in the isolation and characterization of genes coding for several proteins associated with MDR, including P-glycoprotein (P-gp), the multidrug resistance associated protein (MRP1), the lung resistance protein (LRP), and, more recently, the breast cancer resistance protein (BCRP). These transmembrane proteins cause MDR either by decreasing the total intracellular retention of drugs or redistributing intracellular accumulation of drugs away from target organelles. These proteins are expressed at varying degrees in different neoplasms, including the AIDS-associated non-Hodgkin lymphoma and Kaposi sarcoma and are generally associated with poor prognosis. Several MDR-reversing agents are in various stages of clinical development. First-generation modulators such as verapamil, quinidine, and cyclosporin required high doses of drugs to reverse MDR and were associated with unacceptable toxicities. Second- and third-generation MDR inhibitors include PSC 833, GF120918, VX-710, and LY335979, among others. Limitations to the use of these modulators include multiple and redundant cellular mechanisms of resistance, alterations in pharmacokinetics of cytotoxic agents, and clinical toxicities. Studies to validate the role of MDR reversal in the treatment of various malignancies are underway. A potential use of these agents may be to enhance intestinal drug absorption and increase drug penetration to biologically important protective barriers, such as the blood-brain, blood-cerebrospinal fluid, and the maternal-fetal barriers. The use of MDR modulators with drugs such as the antiviral protease inhibitors and cytotoxics may enhance drug accumulation in sanctuary sites that are traditionally impenetrable to these agents.