The breakpoints used to interpret antimicrobial susceptibility tests should be carefully determined initially, using microbiological, pharmacokinetic, pharmacodynamic, and clinical data, and then reevaluated periodically as changes in bacterial resistance, susceptibility test methods, or antibiotic formulations occur. Throughout the world, different governmental agencies and professional organizations have responsibility for the initial establishment of antibiotic susceptibility breakpoints. In the United States, the National Committee for Clinical Laboratory Standards has a mechanism in place to establish breakpoints initially and to review and publish updates on an annual basis. There should be a continued effort to coordinate both susceptibility testing methods and breakpoint determinations in various parts of the world.