There are three commonly recognized species of Nocardia that cause disease in a large variety of animals including humans. In the United States, pulmonary or systemic nocardiosis in humans caused by N. asteroides is most frequently diagnosed. It should be noted that N. brasiliensis can cause nocardiosis also. In Central and South America mycetomas induced by N. brasiliensis appear to be more prevalent even though N. asteroides can be seen in this type of infection. Sporadic cases of both mycetoma and nocardiosis caused by N. caviae have been reported. These three species of Nocardia appear to be present in the soils of most countries; but N. asteroides is more frequently isolated in the temperate climates whereas N. brasiliensis predominates in the tropical and subtropical regions of the world. No specific geographic distribution has been noted with N. caviae. Infections involving these three species of bacteria are, therefore, worldwide in occurrence. Even though nocardial infections in both humans and animals were recognized as early as 1888, traditionally, they have been considered rare in frequency as compared to many other infectious diseases. This is a perception that probably is not accurate. During the past twenty years, greater diagnostic awareness combined with more intensive medical and veterinary surveillance have resulted in significantly increased recognition of disease caused by these aerobic actinomycetes.