This innovative book identifies and explains the concepts that determine when the EU may act.'Competence' is about more than competence alone: there is also power. Competence is the field in which the EU may act; power is the EU institutions' actual authorisation to act in a field of EU competence. Through a rigorous examination of legal disputes that involve the EU treaties, legislation, case law and archival evidence, this book illustrates how competence and power differ and why recognising this distinction leads to a more nuanced and precise analysis of when the EU may act. This will be of interest to EU law scholars and practitioners, as well as international law specialists.