Multidrug resistance (MDR) to anticancer drugs can be diagnosed in tumors by molecular biology techniques (expression of the MDR1 gene), by immunologic techniques (expression of P-glycoprotein), and by functional approaches (dye exclusion). Numerous studies have tried to correlate the MDR status of tumors to the clinical response to the treatment, but wide discrepancies prevented definitive conclusions. As a consequence, the routine use of these techniques is still not possible, and continuous efforts are needed for their standardization. The development of MDR modulators in the clinical setting is a promising approach that requires rigorous clinical trials, especially with sequential design of phase 2 protocols. Definitive results are still lacking concerning the interest of combining an MDR modulator to standard chemotherapy for resistant cancers.