This paper examines the central roles performed by Members of the House of Commons in Canada, assesses long-standing controversies about how MPs should carry them out, and discusses recent trends in the thinking of both parliamentary scholars and MPs themselves concerning key roles. Part I provides historical background on the emergence of modern roles. It includes a discussion of major factors that have propelled the evolution of traditional roles that continue to provide the basis for expectations about what Members of Parliament should do, and how they should do it. Part II reviews the major current roles, as portrayed in recent research on Parliament. It also examines the assumption that Members of Parliament share basic roles, in the light of differences that have emerged between the government and opposition Members. Part III explores some issues and controversies – both recent and long-standing – about what the roles of Members of Parliament should be. It also examines the way in which assumptions about the key roles of Members of Parliament have provided guidance for parliamentary reform initiatives, and discusses possible future directions.