This review summarizes the characteristics of the ATP-binding cassette, subfamily G (ABCG family), which has five members: ABCG1, ABCG2, ABCG4, ABCG5, and ABCG8. The members consist of a single ABC cassette in the amino terminal followed by six putative transmembrane domains, and to become functionally active, they form homo- or obligate heterodimers. Except for ABCG2, the members of the ABCG family play an important role in efflux transport of cholesterol. Mutations causing a loss of function of ABCG5 or ABCG8 are associated with sitosterolemia characterized by accumulation of phyto- and shellfish sterols. Unlike other members, ABCG2 is not involved in cholesterol efflux, but it exhibits broad substrate specificity to xenobiotic compounds. ABCG2 confers cancer cells resistance to anticancer drugs and plays a critical role in the pharmacokinetics of drugs in the clearance organs and tissue barriers. ABCG2 is also associated with a subpopulation phenotype of stem cells. Genetic polymorphisms of ABCG2 have been suggested to account for the interindividual differences in the pharmacokinetics of drugs.