Restitution is the body of law concerned with taking away gains that someone has wrongfully obtained. The operator of a Ponzi scheme takes money from his victims by fraud and then invests it in stocks that rise in value. Or a company pays a shareholder excessive dividends or pays them to the wrong person. Or a man poisons his grandfather and then collects under the grandfather’s will. In each of these cases, one party is unjustly enriched at the expense of another. And in all of them the law of restitution provides a way to undo the enrichment and transfer the defendant’s gains to a party with better rights to them.