This standard provides protocols and related quality control parameters for antimicrobial susceptibility testing of mycobacteria, Nocardia spp., and other aerobic actinomycetes. Clinical and Laboratory Standards Institute standard M24—Susceptibility Testing of Mycobacteria, Nocardia spp., and Other Aerobic Actinomycetes includes susceptibility testing procedures for Mycobacterium tuberculosis complex (MTBC), clinically significant slowly and rapidly growing mycobacterial species, Nocardia spp., and other aerobic actinomycetes. Also included in this standard are recommendations for selecting agents for first-line and second-line drug testing, organism group-specific methodologies, reporting recommendations, and organism quality control criteria. Recommendations regarding agent selection for testing mycobacteria are based primarily on published guidelines. For testing MTBC, M24 recognizes agar proportion as the reference methodology on which all other methodologies are based. In addition, this standard includes recommendations for using commercial broth susceptibility methods with shorter incubation times, which are now in widespread use for MTBC susceptibility testing, and information on molecular methods for detecting drug resistance and their integration with culture-based methods.