Antimicrobial susceptibility test breakpoints are necessary for the correct clinical interpretation of quantitative or qualitative susceptibility test values of antibacterial agents. Breakpoints are selected by regulatory bodies or professional groups (eg, the National Committee for Clinical Laboratory Standards) based on an extensive review of the microbiological, pharmacokinetic, and clinical data applicable to each drug. No precise formula for selection of optimal breakpoints has been established. Instead, the process involves a review of all existing data and is influenced by the perspectives of the individuals charged with selecting the breakpoints. The interpretive breakpoints may differ somewhat by country with some antibiotics because of regional or national decisions on dosing regimens. Breakpoints also influence the selection of empiric therapy and development of local, regional, or national treatment guidelines by classifying the percentage of previously tested organisms that were susceptible to various antibiotics. Thus, antibiotic breakpoints affect the care of individual patients and provide epidemiologists and policy makers with data regarding trends in antibiotic resistance.