Direct Effect in EU Law is the first book in English to thoroughly examine this revolutionary legal principle, tracing direct effect's evolution throughout the multiform process of European integration. A founding doctrine of EU constitutional law and the driving force of the EU legal system, direct effect enables individuals and companies to invoke their rights before domestic authorities and the Court of Justice of the EU (CJEU). Yet, while the doctrine of direct effect remains the backbone of the EU legal system, it lacks clear legal framing by the CJEU. Emphasizing the need for the CJEU to fully articulate and systematize direct effect as to its core components and consequences, the book advocates for an innovative understanding of such principle that acknowledges its transformative impact on EU law. It bridges theory and practice, drawing upon the CJEU rulings dating from the 1950s up to 2024. The book reconstructs direct effect beyond the doctrine originating from Van Gend & Loos, and ultimately puts forward solutions for its principled comprehension and enforcement.