Action theorists routinely take actors’ ability to act for granted. A theory of agency is required to explain how such an ability is acquired and maintained. Such an approach involves recognising that ‘action’ is not the opposite of ‘behaviour’ but simply a special form of behaviour, and hence that a behavioural perspective needs to be incorporated into an action or interpretivist paradigm. Two common objections to such a proposal are then considered. The first is that ‘behaviour’ is too insignificant a portion of human conduct to be of concern to sociologists. The second that behavioural and action theory perspectives are incommensurate, as a behavioural analysis of human conduct ‘from the outside’ is incompatible with an ‘interpretivist’ analysis ‘from the inside’. It is demonstrated that neither claim is tenable given that actors are also self-observers, acting as ‘lay behaviourists’ in applying an understanding of S-R to achieve their goals. In addition, voluntarism does not preclude action being understood as a response to a stimulus, or—crucially—a response to the meaning of a stimulus. It is concluded that a behavioural perspective supplies just the theory of agency that is missing from most theories of action.