The Robust Federation offers a comprehensive approach to the study of federalism. Jenna Bednar demonstrates how complementary institutions maintain and adjust the distribution of authority between national and state governments. These authority boundaries matter – for defense, economic growth, and adequate political representation – and must be defended from opportunistic transgression. From Montesquieu to Madison, the legacy of early institutional analysis focuses attention on the value of competition between institutions, such as the policy moderation produced through separated powers. Bednar offers a reciprocal theory: in an effective constitutional system, institutions complement one another; each makes the others more powerful. Diverse but complementary safeguards – including the courts, political parties, and the people – cover different transgressions, punish to different extents, and fail under different circumstances. The analysis moves beyond equilibrium conceptions and explains how the rules that allocate authority are not fixed but shift gradually. Bednar’s rich theoretical characterization of complementary institutions provides the first holistic account of federal robustness.