Pseudomonas aeruginosa displays three active efflux systems, made of three components: an inner membrane protein acting as a proton motive pump, a periplasmic linking protein and an outer membrane protein acting as an efflux porin. All three proteins are encoded by genes organised as an operon, with a regulator gene in the vicinity. Efflux systems produce phenotypes of multidrug resistance. The constitutively produced system MexAB-OprM generates intrinsic resistance to most beta-lactams, quinolones, tetracycline, chloramphenicol, trimethoprim and sulfamethoxazole; in addition, genetic derepression causes acquired resistance to the same drugs. The non constitutive MexCD-OprJ and MexEF-OprN can be expressed by mutation, producing then slightly different multidrug resistant phenotypes. Efflux systems are responsible for low level resistance, and seem to promote selection of mutations responsible for higher level resistance. The multidrug resistance associated to active efflux systems should be taken into account in our antibiotic policies, both at the individual and at the institutional level.