This Companion provides a broad, historically informed introduction to the study of the US constitutional system. In place of the usual laundry lists of cases, doctrines, and theories, it presents a picture of the constitutional system in action, with separate sections devoted to constitutional principles, organizational structures, and the various legal and extra-legal 'actions' through which litigators and average citizens have attempted to bring about constitutional change. Finally, the volume covers a number of subjects that are rarely discussed in works aimed at a general audience, but which are critical to ensuring that constitutional rights are honored in the day-to-day lives of citizens. These include standing and causes of action, suits against officeholders, and the inner workings of the Foreign Intelligence Surveillance Court (FISC). This Companion places present-day constitutional controversies in historical context, and offers insights from a range of disciplines, including history, political science, and law. Offers an accessible, interdisciplinary, and historically informed introduction to the study of American constitutionalism. Karen Orren is Distinguished Professor of Political Science at University of California, Los Angeles. Her previous books include Belated Feudalism (Cambridge, 1992) and The Search for American Political Development (Cambridge, 2004), with Stephen Skowronek. Her most recent book is The Policy State (2017). John W. Compton is Associate Professor of Political Science at Chapman University, California. He is the author of The Evangelical Origins of the Living Constitution (2014).