The microbiologists use the term corynebacteria to describe aerobically growing, asporogenous, irregularly sharped gram-positive rods. They comprise strictly aerobic bacteria isolated from environment as well as preferentially anaerobic bacteria found in clinical specimens. A large part of these bacteria is considered as commensal of skin and mucous membranes. This group of organisms has recently been subjected to considerable taxonomic revisions, which have resulted in the proposal of several new species, many of them representing previous Centers for Diseases Control coryneform groups. Moreover, recent investigations demonstrated the existence of a pathogenic role for some of them. These bacteria comprise well-known pathogens such as C. diphtheriae responsible for diphtheria, Actinomyces spp. responsible for actinomycosis and Arcanobacterium haemolyticum recovered from pharyngitis, but other corynebacteria were related to particular infections. For example, the lipophilic and antibiotics multiresistant species Corynebacterium urealyticum and C. jeikeium were found to be responsible for urinary tract infections and septicemias, respectively. The recently described species Turicella otitidis was found to be implicated in otitis media and C. seminale were recovered from genital specimens of male patients. Implantation of material devices, use of broad-spectrum antibiotics led to an increase of sepsis due to the species C. jeikeium and C. amycolatum. Many of the new Actinomyces species grow well under aerobic conditions and are often implicated in various abscesses. Moreover an increase of immunocompromised patients led to the development of infections due to the aerobic actinomycete Rhodococcus equi. The association of some corynebacteria with particular diseases should prompt the microbiologist to identify these bacteria when they are encountered in a pathogenic situation. Identification of the major part of corynebacteria isolated from clinical specimens can now be achieved by using recent schemes.