Abstract This chapter outlines the history of One Health. The first section of the chapter analyses intersections between human and animal health in the premodern era, to reveal how deeply animals and animal health were embedded within human medicine. The second section extends from the late 18th-century foundation of the veterinary profession until the turn of the 20th century. It tracks the evolving relationship between the veterinary and medical professions, and how, as scientific ideas and practices changed, new links were forged between human and animal bodies and diseases. The third section extends this analysis into the 20th century, focusing particularly on the changing status of animals within medical research, and on international efforts to develop comparative medicine and veterinary public health. The conclusion reflects on the importance of these findings for history and for One Health today.