After decades of confusion about their microbiologic classification and clinical significance, the nondiphtheria corynebacteria have emerged as important pathogens. Although isolation of these organisms may represent contamination with skin flora, several species, including Corynebacterium ulcerans, Corynebacterium pseudotuberculosis (Corynebacterium ovis), Corynebacterium haemolyticum, Corynebacterium pseudodiptheriticum, Corynebacterium equi, Corynebacterium bovis, Corynebacterium xerosis, and corynebacteria of group JK, clearly cause disease in humans. Most of these organisms infect animals, which are the source of human infection with some species. Some nondiptheria species of Corynebacterium produce recognizable clinical syndromes such as granulomatous lymphadenitis, pneumonitis, pharyngitis, cutaneous infections, and, most commonly, endocarditis. Certain species infect healthy hosts, while others predominantly attack immunocompromised individuals. Several species produce toxins, including a diphtheria-like toxin, a dermonecrotic toxin, and a soluble hemolysin. A microbiologic scheme of identification of the genus Corynebacterium and its major defined species is presented.