The chapters in this volume consider national-level evidence for the operation of Duverger’s law in the world’s largest, longest-lived and most successful democracies of Britain, Canada, India and the United States. One set of papers involves looking at the overall evidence for Duverger’s Law in these countries; the other set deals with evidence for the mechanical and incentive effects predicted by Duverger. The result is an incisive analysis of electoral and party dynamics.