Rickettsial diseases have had a profound impact on human lives for centuries but were poorly understood prior to the beginning of the 20th century. The study of the pathogens is challenging and requires an intimate understanding of arthropod vectors, mammalian hosts, and human nature. This chapter focuses on rickettsial infections of public health importance to humans, primarily focusing on rickettsiae in the United States. It also briefly addresses the global importance of rickettsial pathogens. An understanding of the epidemiology of rickettsial diseases requires not only a look at contemporary public health issues, but also an understanding of the history of these infections and classic approaches to control and prevention. Since the early investigations of Ricketts and others into the origins and ecology of Rocky Mountain spotted fever (RMSF), public health interest in rickettsiae has remained strong in the United States. The chapter concentrates on the historical and current epidemiology and public health importance of rickettsial pathogens known to occur in the United States. R. parkeri infection can be difficult to distinguish from infection with Rickettsia rickettsii, as extensive serologic cross-reactivity occurs between this and other spotted fever group rickettsial (SFGR) species. From 2000 to 2007, 3,126 cases of presumed Ehrlichia chaffeensis and an additional 824 cases classified as "Ehrlichiosis Undetermined/Unspecified/Other Agent (UUOA)" were reported within the United States. Most of these Ehrlichiosis UUOA cases were probably E. chaffeensis or Anaplasma phagocytophilum cases that could not be properly classified.