Breast cancer resistance protein (BCRP/ABCG2) belongs to the ATP-binding cassette (ABC) transporter superfamily. It is able to efflux a broad range of anti-cancer drugs through the cellular membrane, thus limiting their anti-proliferative effects. Due to its relatively recent discovery in 1998, and in contrast to the other ABC transporters P-glycoprotein (MDR1/ABCB1) and multidrug resistance-associated protein (MRP1/ABCC1), only a few BCRP inhibitors have been reported. This review summarizes the known classes of inhibitors that are either specific for BCRP or also inhibit the other multidrug resistance ABC transporters. Information is presented on structure–activity relationship aspects and how modulators may interact with BCRP.