In this chapter, classification of mycobacteria, M. tuberculosis and nontutuberculous mycobacteria (NTM) are described. The morphology, culture characteristics of M. tuberculosis, virulence in animals, susceptibility to physical and chemical agents, antigenic structure and biochemical properties are described in detail. Further, microbiological basis of pathogenesis of TB, Koch’s phenomenon are described. Principles underlying laboratory diagnosis are dealt with.