Pseudomonas aeruginosa is an opportunistic human pathogen characterized by an intrinsic resistance to multiple antimicrobial agents and the ability to develop high-level (acquired) multidrug resistance during antibiotic therapy. Much of this resistance is promoted by highly homologous three-component efflux systems of broad substrate specificity, of which four have been identified to date. These include MexA-Mexs-OprM and MexX-MexY-OprM, which are expressed constitutively in wild type cells and, thus, provide for intrinsic multidrug resistance, and MexC-MexD-OprJ and MexE-MexF-OprN, whose expression so far has only been seen in acquired multidrug resistant mutant strains. Additional homologues of these efflux systems are identifiable in the recently released genome sequence, though their roles, if any, in antimicrobial efflux are unknown. These tripartite pumps are composed of an integral cytoplasmic membrane drug-proton antiporter of the resistance-nodulation-cell division (RND) family of exporters, a channel-forming outer membrane efflux protein (or outer membrane factor [OMF]) and a periplasmic membrane fusion protein (MFP) that links the other two. In addition to a number of antimicrobials of clinical significance, these pumps also export dyes, detergents, disinfectants, organic solvents and acylated homoserine lactones involved in quorum-sensing. While the natural functional of these pumps remains undefined, the fact that they contribute to antimicrobial resistance in P. aeruginosa makes them reasonable targets for therapeutic intervention.