Birks’ unjust enrichment formula was intended to provide a common descriptive structure to all the instances where there was recovery. He did not, however, engage in an analysis of the various reasons why courts awarded restitution. My thesis seeks to fill this gap. I argue that without such an account Birks work is incomplete. According to Birks, for example, money and services both amounted to enrichments and so should be considered together. But there are some differences and similarities between money and services. In order to be able to group them together Birks needs to be able to say that the reasons for giving recovery in money and service cases are similar enough that they can be grouped together. The same goes for all the unjust factors. The point is, the generalisation that Birks sought to do, can only properly be done if one is attuned to the reasons why recovery is granted in each of those cases. If the reasons are similar then the generalisation makes sense. But if they are not then it does not make sense to so generalise. The argument of the thesis is that there three relevant principles to justifying unjust enrichment: the Property Principle, the Benefit-Burden Principle and the Autonomy Principle. The Property Principle states that one should not have property belonging to another. The Benefit-Burden Principle states that if one takes a benefit then one must bear the associated burdens; to put it more colloquially: you have to take the rough with the smooth. These first two principles provide reasons for considering a situation to be defective and the last principle provides a constraint for the operation of the first two. It is there to ensure that the imposition of liability will not unduly affect the autonomy of the defendant. Based on that the thesis proposes that the scope of the unjust enrichment formula be trimmed down to only cover defective transfers of money and other assets. For the other cases, a different analytical structure is needed. This is because the reasons for recovery in those cases are different.