The identity and existence of a loss-based defence in the law of unjust enrichment is disputed. Widely known as 'passing on', but better identified as 'disimpoverishment', this defence has generated confusion and disagreement across and within England, Australia, Canada and the United States of America. This book seeks to address these problems in three ways. First, by providing a solution to the defence's terminological problems and presenting a coherent picture of the current state of the law. Secondly, by examining whether a defendant's unjust enrichment can be said to have come 'at the expense of' a claimant when a third party has borne the cost of that enrichment. Put another way, whether awards of restitution are, or should be, restricted by the value of a claimant's loss. And finally, by analyzing the reasons in favour of accepting or rejecting a loss-based defence in the law of unjust enrichment. Numerous scholarly textbooks and law journals have devoted space to these issues. This work, however, has tended to focus narrowly on either particular cases or sets of issues. This book seeks to address this deficiency by collating, and providing total coverage of, the controversies and questions pertaining to a loss-based defence in the law of unjust enrichment. This work will be essential reading for anyone interested in the law of restitution, and in its relationship with other areas of private law.