Sarcoidosis is a protean disease with strikingly varying characteristics in different climates, in different racial groups, and more recently realized, in different occupations. Genetic studies on familial clustering reveal multiple genetic variations, probably followed by environmental triggers. The epidemiology of the disease is discussed. Systemic involvement is described, including chest disease staging and extrapulmonary disease, particularly neurological involvement. The clinical characteristics of ocular sarcoidosis contributing to diagnosis have been clarified by an International Workshop and are described in the context of variable ocular manifestations, which may affect any part of the eye.