Why are some countries democratic and others mired in endless dictatorship? Why Democracies Develop and Decline, a wide-ranging volume by leaders of the Varieties of Democracy (V-Dem) project, overviews and reexamines the extensive literature on this question. The approach here is very data-forward, rather than attempting to develop new theory on how and why democratic change happens. A big advantage is the authors’ use of the excellent V-Dem dataset, which extends back to 1789 and even covers pre-independence colonies. After an introduction and descriptive chapter on historical trends in democracy, the book’s heart is a series of chapters on five clusters of explanations for democracy: long-run factors (focusing on geography and colonial history), international, economic, politico-institutional, and social. For each, the authors summarize prior work and then present a range of empirical tests to see which explanations hold up. Careful adjustments are made to the empirical models in each chapter, but the authors generally compare predictions of levels, upturns, and downturns on a continuous democracy index. Many of the findings are novel and surprising. For instance, Chapter 3 identifies distance to natural harbors as a key long-run factor explaining historical democracy levels. A final chapter attempts to put the various factors in a causal ordering, distinguishing long-run from more proximate factors. The comprehensive summaries of prior research and illustrations of how to improve empirical analysis makes this an ideal book for graduate students and academics interested in democratization and democratic survival.