Which rights and obligations arise from the EU principle prohibiting unjust enrichment? This is the first publication to thoroughly examine the consequences this principle has # or may have # for private law relationships. An illuminating analysis, bearing both academic and practical importance. As the interplay between EU law and national private law intensifies, the question arises how the EU principle prohibiting unjust enrichment plays into various legal relationships involving one or more individuals. Unjust enrichment in European Union law takes a pioneering step in addressing this pressing issue. The author puts forward a compelling analysis, taking into account the functions of unjust enrichment in a number of national law systems and the functions of general principles of EU law, as well as case law of the Court of Justice of the EU. For analytic purposes, links are identified between EU causes of action based on undue payment, unjust enrichment and unlawful act, respectively. This is followed by a discussion whether or not such actions should be founded on violation of an EU provision having direct (horizontal) effect. Insight into the possible consequences of the EU principle prohibiting unjust enrichment has both academic and practical importance. The reader gains a deeper understanding of how the Court of Justice may further develop EU law on the basis of private-law principles. The study illuminates which rights individuals may derive from such legal principles and # if they can do so # under which circumstances.