Human ehrlichioses are emerging tickborne infections. "Human ehrlichiosis" describes infections with at least 5 separate obligate intracellular bacteria in 3 genera in the family Anaplasmataceae. Since 1986, these agents and infections (human monocytic ehrlichiosis [HME], caused by Ehrlichia chaffeensis; human granulocytic anaplasmosis [HGA], caused by Anaplasma phagocytophilum; and human ewingii ehrlichiosis, caused by Ehrlichia ewingii) are the causes of most human ehrlichioses. Their prevalence and incidence are increasing where the appropriate tick vectors are found. The diseases generally present as undifferentiated fever, but thrombocytopenia, leukopenia, and increased serum transaminase activities are important laboratory features. Despite clinical similarities, each disease has unique features: a greater severity and a higher case-fatality rate for HME and a higher prevalence of opportunistic infections for HGA. Once an ehrlichiosis is suspected on historical and clinical grounds, doxycycline treatment should be initiated concurrently with attempts at etiologic confirmation using laboratory methods such as blood smear examination, polymerase chain reaction, culture, and serologic tests.