This chapter focuses on parliamentary regimes. Parliamentarism, in its various forms, has the distinction of being the most widespread type of democratic government in the world. The chapter recounts the origins of parliamentary government in England, the cradle whence it was adopted and modified, in many different ways, in the rest of Europe, and was exported in tandem with the British Empire’s expansion. It next contrasts parliamentarism with presidentialism, highlighting the higher levels of executive-legislative comity in the former, and showing that the most consequential function of legislative assemblies under parliamentarism is not to legislate or even to scrutinize, but to make or break governments, which are typified by a countervailing power to convene or dissolve assemblies. The chapter then underscores the reality that parliamentary systems are not monolithic. It exhibits the variation and diversity in the universe of parliamentary regimes in the provisions for the head of state, the head of government and the cabinet, parties and elections, parliamentary control of government, and judicial review of administrative action. Finally, it concludes with a summary of major findings.