This chapter presents a glimpse on nontuberculous mycobacterial (NTM) infections. NTM refers to a group of environmental mycobacteria that are found in the soil or natural or treated water. Most identified species are nonpathogenic. Infection can occur in immune-competent as well as immunocompromised patients. Most infections occur among young adults and elderly persons. Because a single specific test to differentiate NTM colonization from active infection is not available, diagnostic criteria have been developed to aid in determining which patients require treatment. Tables on diseases caused by common NTM species, diagnostic criteria for NTM lung disease, and treatment of nontuberculous mycobacteria infections in children are provided in this chapter, which are self-explanatory and very useful for readers. Mycobacterium avium complex (MAC) is the most common cause of lung disease and is acquired by inhaling aerosolized microorganisms from colonized soil and water. Treatment of MAC infection generally includes a combination of macrolide (clarithromycin or azithromycin), ethambutol and rifamycin. Mycobacterium kansasii is the second most common NTM species causing lung disease. The rapidly growing mycobacteria (RGM) is defined by their brief growing period in culture media, within 7 days.