Ehrlichial diseases of dogs currently are recognized to be caused by Ehrlichia canis, Ehrlichia platys, Ehrlichia equi, and Ehrlichia risticii. Information on each disease's history, transmission, pathogenesis, clinicopathologic findings, diagnosis, therapy, and prevention is presented in this article. Ehrlichia canis is the most common cause of clinical illness. Case reports are included to illustrate common presentations of Ehrlichia canis infections.