Antibiotics taken up into gram-negative bacteria face two major diffusion barriers, the outer and cytoplasmic membranes. Of these, the former has been most studied and is discussed in detail here. Evidence from antibiotic MIC studies on porin-deficient mutants compared with their porin-sufficient parent strains has provided strong support for the proposal that some antibiotics, particularly beta-lactams, pass across the outer membrane through the water-filled channels of a class of proteins called porins. Nevertheless substantial evidence has accumulated for the importance of non-porin pathways of antibiotic uptake across the outer membranes of gram-negative bacteria. Examples discussed include the uptake of polycationic antibiotics via the self-promoted pathway, the uptake of hydrophobic antibiotics in some bacterial species and in mutants of others via the hydrophobic pathway, and the possible importance of poorly understood non-porin pathways of uptake of a variety of antibiotics. Other potential barriers to diffusion, including the cytoplasmic membrane, are briefly discussed.