beta-lactams are the antibiotic compounds most widely used against hospital and community acquired infections. However, resistance has emerged in both Gram-positive and Gram-negative bacteria, limiting their therapeutic efficacy. The choice of appropriate treatment depends on analysis of susceptibility data that indicates a specific mechanism of resistance. Correct interpretation of susceptibility tests permits a rational approach to the resistance problem and selection of alternatives for treatment. The laboratory must first be able to identify accurately microorganisms to the species level and then test a minimum of relevant antimicrobials. beta-lactam resistance in Enterobacteriaceae is mainly due to the production of plasmid or chromosomal encoded beta-lactamases. In Gram-negative non-fermenting bacteria, impermeability and efflux are relatively more important to the treatment selected. In Gram-positive bacteria, resistance mechanisms can involve changes in penicillin-binding proteins (PBPs), production of new PBPs or synthesis of beta-lactamases. The range of therapeutic options must be based on the current status of local resistance mechanisms.