This chapter focuses on the evolution of systems of constitutional justice since 1787. It first provides an overview of key concepts and definitions, such as constitution, constitutionalism, and rights, before presenting a simple theory of delegation and judicial power. In particular, it explains why political elites would delegate power to constitutional judges, and how to measure the extent of power, or discretion, delegated. It then considers different kinds of constitutions, rights, models of constitutional review, and the main precepts of ‘the new constitutionalism’. It also traces the evolution of constitutional forms and suggests that as constitutional rights and review has diffused around the world, so has the capacity of constitutional judges to influence, and sometimes determine, policy outcomes.