ABSTRACT Nontuberculous mycobacteria (NTM) are emerging pathogens that affect both immunocompromised and immunocompetent patients. The development of molecular methods has allowed the characterization of new species and the identification of NTM to the precise species and subspecies levels. The incidence and prevalence of NTM lung disease are increasing worldwide, and this syndrome accounts for the majority of clinical cases of NTM disease. Common causative organisms of pulmonary infection are the slowly growing mycobacteria Mycobacterium avium complex and Mycobacterium kansasii and the rapidly growing mycobacteria, including Mycobacterium abscessus complex. NTM lung disease often affects elderly people with chronic lung disease and may be a manifestation of a complex genetic disorder determined by interactions among multiple genes, as well as environmental exposures. To be diagnosed with NTM lung disease, patients should meet all clinical and microbiologic criteria, but the decision to start treatment is complex, requiring careful individualized analysis of risks and benefits. Clinicians should be alert to the unique aspects of NTM lung disease, including the need for proper diagnosis, the availability of advanced molecular methods for species and subspecies identification, and the benefits and limitations of recommended treatments.