Quinolones are useful synthetic antimicrobial agents in the clinical settings with broad specrum, potent efficacy, good bioavailability, and excellent safety. Therefore, development and increase of quinolone-resistance among clinically isolated pathogens are a worrisome problem. Recently, increasing numbers of plasmid-mediated quinolone resistance (PMQR) among clinical isolates of Enterobacteriaceae have been reported. PMQR consists of Qnr family as a protective determinant of bacterial drug-target enzymes, Aac (6')-Ib-cr as a drug-modifying enzyme, and plasmid-mediated quinolone efflux pumps. In this review, biological features of PMQR determinants including discovery, phylogeny and presumable physiological role, as well as their clinical contribution of the acquisition and development of quinolone resistance, are discussed.