Two Hepatozoon spp are recognized as parasites of domestic dogs in the United States, H. canis and H. americanum. H. canis was first described in India in 1905 and has been documented in many areas of the world, although not definitively identified in North America until recently. H. americanum, causing American canine hepatozoonosis, was first documented in a coyote in 1978 and is now considered an emerging etiologic agent of disease in domestic dogs throughout the United States. The authors review current knowledge of canine hepatozoonosis caused by H. canis and H. americanum and elaborate on more recent research findings.