By the mid-nineteenth century in Britain there were numerous groups on the fringe of medicine. Some were clearly separated from orthodoxy, while others represented medical heresies arising within the regular ranks. Established medicine was formalising its professional structure and needed urgently to define its boundaries. This paper examines the basis on which lines of demarcation were drawn between orthodoxy and three examples of the medical fringe—hydropathy, herbalism and the marketing of ‘patent medicines’.