This doctoral research investigates prime ministerial predominance In Britain and Australia. It makes an important contribution to the study of prime ministerial leadership, using comparative research methods. The research is located in the emerging field of comparative political leadership and uses case study analysis of two prime ministers in similar political systems: Tony Blair (1997-2007) and John Howard (1996-2007). The research draws on two strands of literature which tend not to be used together. The core executive model favoured by institutionalists considers the prime minister as one of many interdependent actors in a model of executive government whereby power is dispersed. It is the role and function of these actors which explain political change. This contrasts with the individualised or personalised model which focuses on a single leader and the impact of agency. Seeing the prime minister as the predominant actor with greater n operating within the core executive model, enables us to progress beyond the disputed territory.