The purpose of this review is to familiarize a broad range of medical professionals with a relatively new and growing problem of infections caused by mycobacteria other than M. tuberculosis and M. leprae. There are at least 60 mycobacterial species that have been identified as causative agents of diseases in humans. They are all environmental bacteria, and they are not transmitted from person to person. The usual source of infection is water, soil, and aerosols developed from these sources. The probability of contracting such a disease depends not only on the closeness of interaction with an environment containing an enhanced concentration of bacteria but also and foremost on the sensitivity of an individual to these infections, which may depend on the state of immunity and other so-called predisposing conditions. Therefore, these infections are often referred to as opportunistic, and the group of organisms causing them are usually referred to as nontuberculous mycobacteria (NTM). This review addresses in a condensed form various aspects of these infections, including bacteriology, clinical manifestations, diagnosis, and therapy. The reader will be able to find more details on each of these topics in several reviews and some original papers cited in this article.