In The Federalist No. 47 James Madison asserted that “accumulation of all powers, legislative, executive, and judiciary, in the same hands…may justly be pronounced the very definition of tyranny.” Indeed, the importance of the separation of powers was so widely accepted by the American public in 1788 that Madison could confidently declare it to be “the sacred maxim of free government.” Today, however, government agencies routinely make, enforce, and adjudicate legally binding rules that have the full force and effect of laws passed by Congress. Such evidence leaves no doubt that there has been a revolutionary shift in the constitutional theory guiding American politics since the time of the American Founding. But how—and why—did this revolution come to be? The answer is to be found in a broad movement known as progressivism that came to dominate both the American academy and government in the late-nineteenth and early-twentieth century.