Across the globe the practice of legitimate opposition is menaced by populists and autocrats. What is lost when legitimate opposition is undermined? This book answers that question, providing the first sustained treatment of legitimate opposition since the Cold War. On the orthodox view, legitimate opposition is quintessentially democratic and was invented with representative government in the late 18th-century. Combining historical and normative argument, I illustrate the untenability of that view. Moreover, I show why normative efforts to reconcile opposition with democracy, undertaken by theorists like Dahl, Robert Goodin, Ian Shapiro and, most famously, J.S. Mill, fail to credibly identify the value of the imperfect, frequently inegalitarian, real-word practice. By focusing on how popular institutions can facilitate or distort citizens’ agency, I provide a novel account of opposition’s value and the moral wrong entailed when it is undermined.