The concentrations achieved by a particular antibiotic in serum and tissues can be predicted from pharmacokinetic data. These predictions must be evaluated in the broader context of the patient's infection, particularly the MIC of the drug for the infecting organism. What is the relevant measurement - the serum concentration or the tissue to serum ratio? Are the serum and/or tissue concentrations achieved at the site of infection sufficiently in excess of the MIC to eradicate the organism? How long are these levels sustained? How often must the drug be given to keep the concentrations sufficiently in excess of the MIC to eradicate the organisms? This presentation will review these questions in the context of the major classes of antibiotics: the beta-lactams, aminoglycosides and quinolones.