This book examines the place of the concept of constituent power in constitutional history, focusing on the legal and institutional implications that theorists, politicians, and judges have derived from it. It shows that constituent power, even though having historically been associated with extra-legality and violations of the constitutional order, has played important functions in the making of determinations of legal validity. Constitutional courts have employed it to justify their jurisdiction to invalidate constitutional amendments that alter the fundamental structure of the constitution and thus amount to a constitution-making exercise. Some governments have recurred to it to defend the legality of the transformation of the constitutional order through procedures not contemplated in the constitution’s amendment rule but considered participatory enough to be seen as equivalent to ‘the people in action’, and these attempts have sometimes been sanctioned by courts. Commentators and citizens have relied on the theory of constituent power to defend the idea that electors have the right to instruct representatives, and that the creation of new constitutions must take place through extra-legislative entities, such as primary assemblies open to all citizens. Several Latin American constitutions explicitly incorporate the theory of constituent power and allow citizens, acting through popular initiative, to trigger constitution-making episodes that may result in the replacement of the entire constitutional order. Building on these findings, the book ultimately develops a distinction between sovereignty and constituent power and argues that even a constitution-making body can be made legally subject to the conditions arising from a constituent referendum.