Anticancer drugs interact directly with their molecular targets in cancer cells for effective cancer chemotherapy. The direct interaction between drug and cancer cell depends on the pharmacokinetics, which consists of absorption, distribution, metabolism, and excretion phases. In the excretion phase, ATP-binding cassette (ABC) transporters are the most important proteins in cell membranes. The ABC transporters export drugs out of cells by ATP-dependent energy, leading to drug resistance with reduced concentrations of intracellular drugs. In addition, the transporters sequestrate intracellular drugs into membrane vesicles in cytoplasm, also resulting in drug resistance. On the other hand, they are also involved in drug absorption. To date, 48 ABC genes have been isolated and classified into the seven groups of ABCA to ABCG. Among them, P-glycoprotein/ABCB 1, MRP 1/ ABCC 1, MRP 2/ABCC 2, MRP 3/ABCC 3, and BCRP/ABCG 2 strongly confer anticancer drug resistance, and they have different substrate drugs. Interestingly, recent molecular-targeted drugs, such as imatinib and gefitinib, were very recently found to be substrates for P-glycoprotein and/or BCRP. Additionally, polymorphism of ABC genes affects pharmacokinetics, drug effectiveness, and adverse events. Thus, ABC transporters are clinically important molecules, and much information is needed in the clinic.