Abstract The law of restitution is said to be governed by the principle of preventing unjust enrichment. But the distinction between just and unjust enrichment has puzzled generations of judges, lawyers, and scholars. Prominent attempts to do so without relying on an open-ended intuitionist sense of right and wrong fall apart upon scrutiny. This chapter argues that private law’s commitment to relational justice offers the answer and demonstrates this claim through the study of two major paradigmatic categories of the law of restitution: the law of mistaken payments, often described as restitution law’s “core case,” and the doctrines governing self-interested conferral of benefits.