This book examines the role of unjust enrichment in the contractual context, defined as contracts which are terminated for breach, or subsisting, or unenforceable. The book makes three claims in relation to the orthodox common law account of restitution (founded on unjust enrichment) in the contractual context. Firstly, the orthodox account correctly proceeds on the basis that the restitutionary claim in the contractual context is founded on an independent cause of action in unjust enrichment, rather than some equitable notion of unconscientiousness or the law of contract. Secondly, the book departs from the orthodox account by rejecting the unjust factors approach and endorsing the absence of basis approach for the law of unjust enrichment. Finally, the book argues that the right to restitution in the contractual context should be determined by the conditionality of the transfer of the benefit rather than a requirement such as the termination of the contract, as the orthodox account dictates. To that end the book proposes the following model, under which the right to restitution in the contractual context is determined by the resolution of the following two questions: Was the transfer of the benefit (eg of money or services) conditional? Was there a qualifying failure of condition? A condition can be, and often is, the other contracting party's counter-performance, but it may also be an event not promised by either party. What qualifies as a failure of condition depends on the type of contract in question. This book identifies two types of contracts, namely those which are apportioned (eg instalment contracts) and those which are unapportioned. It is only in relation to the latter that termination is required. It is a particular strength of the book that it is underpinned by detailed and original historical analysis which makes a novel and distinct contribution to the history of the laws of unjust enrichment and contract. This book provides groundbreaking, historic