The relationship between populism and democracy is a hotly debated topic. Some believe that populism is inherently bad for democracy because it is anti-pluralist and confrontational. Others argue that populism can reinvigorate worn-out democracies in need of an infusion of greater popular participation. This book advances this debate by examining the empirical relationship between populism in power and democracy. Does populism in power always lead to regime change, that is, the demise of democracy? The answer is no. The impact of populism on democracy depends on the variety of populism in power: the worst outcomes in democratic governance are found under unconstrained populism. This book discusses the conditions that explain how populism becomes unconstrained, and advances a dynamic theory of change that shows how the late victories of populists build on early ones, resulting in greater power asymmetries. The book analyzes five populist presidencies in the Andes. In four of them (Bolivia, Ecuador, Peru, and Venezuela), populism became unconstrained and regime change followed. In one case, Colombia, populism in power was contained and democracy survived. The concluding chapter places the Andean cases in comparative perspective and discusses how unconstrained populism in other cases (Nicaragua and Hungary) also lead to the end of electoral democracy. Where populism in power was constrained (Honduras and the United States), regime change did not materialize. This book advances a theory of populism that help us understand how democracies transition into non-democracies. To that extent, the book illuminates the processes of democratic erosion in our time.