Chemotherapy is currently one of the most effective ways to treat metastatic cancers. However, of the various mechanisms that are involved in conferring resistance, upregulation of drug efflux ATP-binding cassette (ABC) transporters, such as P-glycoprotein (ABCB1), multidrug resistance protein 1 (ABCC1) and ABCG2, has become a major obstacle to cancer chemotherapy and seriously affects the clinical outcome. To date, at least 15 ABC drug transporters have been identified and characterized to transport and confer resistance to practically the entire spectrum of cancer drugs, causing multidrug resistance (MDR) in cancers. Unfortunately, despite decades of research, there is still no real solution to MDR. This review highlights some of the major findings, the roles and problems associated with MDR-linked ABC drug transporters in metastatic cancers and solid tumors, and the current strategies to improve the clinical outcome in cancer chemotherapy.