To be effective, an antibiotic must achieve therapeutic concentrations at the site of infection. This article is a review on the bioavailability of local and systemic antibiotics. Amongst topical antibiotics, fluoroquinolones and fusidic acid have the best intra-corneal and intra-cameral penetration. Chloramphenicol penetrates the anterior chamber but not always at therepeutic levels. Serum levels are low from topical administration, and haematological toxicity of chloramphenicol eyedrops is still not proven. Amongst systemic antibiotics, the molecules capable of penetrating the eye at therapeutic levels are fosfomycin, imipenem, some of third generation cephalosporins, fluoroquinolones and ureidopenicillins. Intra-ocular penetration of antibiotics is increased by infection, corneal epithelium abrasion, increasing dose frequency or delivery using a biomaterial reservoir (soft contact lenses or collagen schields). Antibiotic-antiinflammatory associations can make infected sites more accessible to antibiotics. The choice of a treatment must take in consideration molecules, dosing and route, and be adapted to the germ to kill and the infected site to treat.