The clinical importance of nontuberculous mycobacteria (NTM) has recently been increasing worldwide. In Korea, Mycobacterium avium complex and M. abscessus are the most commonly encoun-tered pathogens of NTM lung disease. When NTM lung disease occurs, it is likely to present in one of two prototypical forms: a fibrocavitary form often affecting older male smokers with chronic obstructive pulmonary disease and nodular bronchiectatic form classically occurring in middle-aged or older women who have never smoked. Since NTM are ubiquitous in the environment, the isolation and identification of causative organisms are mandatory for diagnosis, and some specific diagnostic criteria have been proposed. The decision on whether to treat a patient remains a matter of careful individual evaluation taking into account the NTM species, extent and form of the disease, and overall condition of the patient. Although treatment regimens that include the new macrolides such as clarithromycin or azithromycin are more effective than earlier regimens, treatment of these infections is time-consuming, complicated, and often ineffective. Surgery for localized NTM lung disease may be useful for selected patients. Observation without treatment may be appropriate for some patients with a slowly progressive form of the disease that is expected to be particularly difficult to treat.