Clinical and Laboratory Standards Institute document M23-A3--Development of In Vitro Susceptibility Testing Criteria and Quality Control Parameters; Approved Guideline--Third Edition offers guidance for developing interpretive criteria and quality control ranges for antimicrobial susceptibility tests against aerobic and anaerobic bacteria, and yeasts performed by CLSI antimicrobial susceptibility testing standards. It describes the data utilized by the Antimicrobial Susceptibility Testing and Antifungal subcommittees to establish these interpretive criteria and QC ranges for antimicrobial agents, including MIC distributions against relevant microorganisms, pharmacokinetic (PK) and pharmacodynamic (PD) characteristics, and clinical outcome data. As antimicrobial agents are used in practice, additional experience accrued may be used to reassess interpretive criteria or QC ranges. Users of these guidelines should understand that susceptibility test results cannot predict with absolute certainty clinical outcomes. They should be used along with the best clinical judgment and laboratory support to draw the best conclusions serving the patient.