This book offers an overview of federalism, the separation of powers, and related matters of constitutional structure. It covers such topics as: the lawmaking powers of the national government (including those powers conferred by the Commerce Clause, the Taxing and Spending Clause, the Necessary and Proper Clause, the Enforcement Clauses of the Reconstruction Amendments, and other sources of federal legislative authority); federalism-based "external" constraints on congressional power (including those provided by the anti-commandeering principle, the "equal sovereignty" principle, and principles of state-sovereign immunity); federalism-based limits on state authority (including those imposed by the dormant Commerce Clause, the Article IV Privileges and Immunities Clause, and statutory preemption doctrine); structural constitutional principles concerning governmental entities other than the states (including Native nations, overseas territories, and the District of Columbia); and the horizontal allocation of power across the three branches of the federal government (including with respect to foreign and military affairs, the federal administrative state, the appointment and removal of executive-branch officials, impeachment, presidential and legislative immunities from judicial process, and the powers of the federal courts).