Antimicrobial concentrations at the site of infection are an important predictor of antibiotic efficacy. Pharmacokinetics is a subdivision of pharmacology that describes how drug concentrations change as a function of absorption, distribution, and elimination. As a result, pharmacokinetic concepts can be used by practitioners to prescribe and adjust antibacterial dosage regimens so that concentrations are optimal for the treatment of infectious diseases. This article reviews basic pharmacokinetic principles. It also illustrates through practical application the pertinence of pharmacokinetics to antimicrobial therapy.