Nontuberculous mycobacteria (NTM) are found commonly in respiratory specimens. In many clinical laboratories, the majority of mycobacteria recovered from respiratory samples are NTM. Perhaps this is because NTM are common in the environment. The water that we drink or shower in often contains NTM; consequently, we often find NTM in respiratory secretions. The term colonization has been used when NTM are recovered more than once even though a specific disease cannot be demonstrated. Under these circumstances, it is never quite clear whether a low-grade infection exists or if secretions are simply contaminated by environmental organisms; however, colonization can be discerned from infection by focusing on the elements of making the diagnosis of an NTM-related disease, rather than simply finding the organism in respiratory samples.