This course surveys major developments and disputes in twentieth-century economic thought, beginning with the marginal revolution’s implications for money, utility, welfare, and interest. It examines how Mises, Hayek, market socialists, and later theorists debated whether planning can reproduce the knowledge, prices, and calculation generated by markets. Lessons on externalities, property rights, competition, monopoly, capital, and social choice reveal competing standards for evaluating efficiency and institutions. The course then contrasts Keynesian explanations of unemployment and aggregate demand with monetarism, rational expectations, and real business cycle theory. Growth models connect saving, capital accumulation, innovation, and steady states, while game theory clarifies strategic interaction, cooperation, credible threats, and equilibrium. Public choice and government-debt analysis extend economic reasoning to voters, officials, bureaucracies, interest groups, and future generations. Throughout, Robert Murphy reconstructs influential models carefully while highlighting their assumptions, internal logic, and major Austrian and heterodox criticisms.