Theories about the decline of legitimacy or a legitimacy crisis are as old as democracy itself. Yet, representative democracy still exists, and the empirical evidence for a secular decline of political support in established democracies is limited, questionable, or absent. This lack of conclusive evidence calls into question existing explanatory theories of legitimacy decline. How valid are theories of modernization, globalization, media malaise, social capital, and party decline, if the predicted outcome (i.e. secular decline of political support) does not occur? And which (new) explanations can account for the empirical variation in political support in established democracies? This book systematically evaluates the empirical evidence for legitimacy decline in established democracies, the explanatory power of theories of legitimacy decline, and promises new routes in investigating and assessing political legitimacy. In doing so, the book provides a broad and thorough reflection on the state of the art of legitimacy research, and outlines a new research agenda on legitimacy.