There has been an increasing amount of information published recently about the ability of different antimicrobial agents to penetrate specific areas of the lung. Initially, whole lung concentrations were reported, but this methodology has been refined so that levels are now measured in potential sites of infection, such as the bronchial mucosa, epithelial lining fluid, and alveolar macrophages, as well as in sputum. Distinct differences in the ability to concentrate in these sites have been shown among beta-lactams, quinolones, and macrolides. Considerable intracellular concentration of quinolones and macrolides, but not beta-lactams, occurs. This intracellular accumulation has been used for the treatment of atypical mycobacteria. The potential effects of high antibiotic levels on cellular function have also been investigated. The clinical relevance of these findings for the choice of antimicrobial agent, dosage, formulation (eg, liposomal), and route of administration (eg, nebulized) are increasingly being explored.