If you pay someone under a contract, and get nothing of what was promised in return; if you give someone a generous gift, but only because they exerted an undue influence over you; if you paid too much tax by mistake - should you get your money back? If the answer is yes, then the mechanism is the law of unjust enrichment, a cause of action in the law of obligations, on a par with tort law and contract law. This book stakes out a modern explanation of the law of unjust enrichment, with a structure that makes the subject readily accessible and navigable.