Political parties are ideal subjects for the study of power because they are specific sites in which it is produced and organised, fought over, captured and lost. However, the literature on political parties largely lacks an explicit and systematic theorisation of power as it is exercised and operates in them. As a result, the study of parties has not kept up with developments in theoretical approaches to power and power relations. For example, the failure to recognise how power works through constituting subjects who are empowered as effective agents with appropriate skills and capacities is a major lacuna in the literature. Parties are not only electoral machines or vehicles for personal ambition: they are organisations, complex relations of individuals, rules and rituals. An approach to power in parties should reflect this. To this end, I develop a five-dimensional framework of power which I use to account for political parties in all their complexity. My aim is to introduce some of the more nuanced and sophisticated insights of political theory to the analysis of political parties without dismissing the benefits of some of the more established ways of looking at power. Power is therefore approached as a rich, multi-dimensional concept, derived from diverse intellectual traditions, including behaviouralist, structuralist and Foucauldian accounts. My framework encapsulates individual agency, the strategic mobilisation of rules and norms, rationalisation and bureaucracy, the constitution of subjectivities and the micro-level discipline of bodies. Theory is employed in conjunction with original interview and archive research on the British Labour Party to construct an account of how power operates in party settings. This provides a unique and, I argue, much richer perspective on the exercise and operation of power in political parties than has been offered before.