Among non-fermenting Gram-negative rods, the most clinically important species are Pseudomonas aeruginosa, Acinetobacter baumannii, and Stenotrophomonas maltophilia, which are frequently multiresistant. P. aeruginosa resistance to beta-lactams depends on the production of chromosomal and plasmid-mediated beta-lactamases, altered permeability (loss of OprD porin is related to carbapenem-resistance) and active efflux systems, particularly MexAB-OprM. In aminoglycoside resistant strains, the main mechanism of resistance is the production of modifying enzymes; the efflux pump MexXY-OprM is also involved. Quinolone resistance in P. aeruginosa is related to changes in topoisomerases, altered permeability and efflux pumps. Multiresistance in A. baumannii may be associated with the acquisition of resistant islands carrying different resistant determining factors. Several broad- and extended-spectrum beta-lactamases have been shown in this microorganism, however the main mechanism of resistance to betalactams is the hyperproduction of the chromosomal cephalosporinase (AmpC) related to the presence of the ISAba1 in the promoter region of this gene. The main mechanism of resistance to carbapenems is the acquisition of carbapenemases type B (metallo-β-lactamases) or class D (oxacillinases), however the loss of a porin can also contribute to modulate the final MIC. The resistance to aminoglycosides has been associated with the production of modifying enzymes or the overexpression of efflux pump (AdeABC), whereas the resistance to quinolones is related to changes in the protein targets as well to the overexpression of efflux pump(s). S. maltophilia show resistance to beta-lactams including carbapenems due to the production of two beta-lactamases (L-1 and L-2). Aminoglycoside-modifying enzymes have also been described in this species. In contrast to that observed in other organisms, S. maltophilia resistance to quinolones has been mainly related to active efflux, rather than to targets alterations.