At the beginning of the twenty-first century prime ministers loom larger in the consciousness of their nations than perhaps in any previous era. But how well do we really understand the variables of prime-ministerial performance, and, specifically, why some prime ministers apparently flourish in the role while others wither? This study examines how prime ministers perform as leaders of their governments, parties, and nations. It offers new ways of thinking about prime-ministerial power and leadership, and systematic empirical studies of prime-ministerial leadership practices in four Westminster democracies: the United Kingdom, Canada, Australia, and New Zealand. The volume features contributions from leading political scientists from all of these countries and is organised into three major sections: understanding power in prime-ministerial performance, prime ministers and their parties, and evaluating prime-ministerial performance. Through its collaborative and multifaceted approach the volume demonstrates that there are no hard and fast propositions or rules of thumb to capture what it is that makes us think of some prime ministers as so much more effective than others. Instead it highlights the importance for students of executive government to grasp the contingent interplay between personal, institutional, and contextual factors in understanding and evaluating prime-ministerial performance.