Modern legal systems generally have a category of obligations known variously as restitution, unjust enrichment, unjustified enrichment or some variant of these. Whichever legal system they are found in, they have a common source in Roman law: the stoic idea that enrichment at the expense of another is unjust, obligations quasi ex contractu and the different forms of non-contractual condictiones. This chapter traces the development of the ideas from Justinian to the modern day, picking up ideas of equity along the way. It focuses on the shifting principles underlying the category, typified by its name, rather than its concrete instantiations, aiming to trace the largely unthinking patterns of borrowing from one legal system to another and from one language to another.