In recent years a number of public health, prevention and disease management strategies have emerged that depend on changing health-related behaviours. The definition of those behaviours, indeed of the very idea of behaviour, remains unchallenged in these initiatives, as behaviour is a taken-for-granted concept. Yet the idea of a changeable behaviour is a relatively recent phenomenon and the aim of this paper is to map its emergence and transformation over the last century. Its origins are shown to lie in the first half of the 20th century when it was derived from the ideas of conduct and movement. From mid-century onwards, it has been increasingly construed as being underpinned by a sense of agency and as a legitimate target for healthcare interventions. Finally, in the 21st century it has become stabilized as a core dimension of health and illness.