Many clinically useful antibacterial drugs have intracellular target sites. Therefore, in order to reach their targets, these compounds must be able to cross bacterial outer and cytoplasmic membranes. Considerable information is available on the mechanisms by which antibiotics cross bacterial membranes and, in many cases, it is now possible to define the molecular basis of their uptake. Passage of drugs across the outer membrane of Gram-negative bacteria can occur by diffusion through porin channels (e.g. beta-lactams and tetracyclines), by facilitated diffusion using specific carriers (e.g. albomycin), or by self-promoted uptake (e.g. aminoglycosides and polymyxins). Transfer of antibiotics across the bacterial cytoplasmic membrane is usually mediated by active, carrier-mediated, transport systems normally operating to transport essential solutes into the cell. For example, the antibiotic streptozotocin bears sufficient structural resemblance to N-acetyl-D-glucosamine to be transported by the phosphoenolpyruvate:phosphotransferase system, and D-cycloserine is recognized by the D-alanine, proton motive force dependent transport system. However, in some cases (e.g. tetracycline) although carrier-mediated transport is implied by the observation that drug uptake is energy dependent, the nature of the membrane carrier(s) responsible is unknown. Knowledge acquired from studies on bacterial peptide transport has been successfully used to deliver (or smuggle) amino acid mimetics disguised as peptides into the bacterial cell. These amino acid mimetics, although often poorly transported in their own right, are frequently potent inhibitors of bacterial peptidoglycan or lipopolysaccharide synthesis once they have gained access to the interior of the cell.