Actinomycosis is a chronic, suppurative, granulomatous and spreading disease which is caused by anaerobic bacteria of the family Actinomycetaceae in humans and other homoiothermic animals. Actinomyces israelii is the most common agent of actinomycosis. Other Actinomyces species responsible for actinomycosis are A. odontolyticus, A. meyeri, A. naeslundii and A. viscosus. Actinomycosis is an endogenous infection and is induced by some predisposing factors that introduce Actinomyces species, which are the normal inhabitants of the host, into the mucosa. Based on the site of involvement, the four common forms are cervicofasial, abdominal, toracic and pelvic actinomycosis. Cerebral, cutaneous or disseminated actinomycosis are extremely rare. Purulent matter, sputum, vaginal discharge, fistulae content or tissue biopsy specimens are generally used to diagnose actinomycosis. However, it is very difficult to identify Actinomyces in these samples because of other filamentous and anaerobic bacteria. Furthermore, actinomycosis is frequently undiagnosed or misdiagnosed and thus is not treated correctly due to the number of other fungal infections which may present with similar manifestations. In most cases, definitive diagnosis is made after surgical resection. Thus, it is very important to know the general features of actinomycosis forms and the diagnostic methods. In this review, the clinical features of actinomycosis forms, general features of Actinomyces species that are responsible for human actinomycosis, the diagnostic tools for actinomycosis and the treatment of actinomycosis are discussed in detail in the light of the literature.