This book examines the justification for restitutionary awards for a failure of condition and considers when, given this justification, such awards should be made. It argues that such awards give effect to, but do not enforce, an agreement between the parties. In recent years, this question has been the focus of more scrutiny and scepticism on the part of private law scholars. Adding to this scepticism, this study promotes a new ‘agreement theory’ of failure of condition, which offers a much less controversial way of understanding the role of restitution in such failures. Innovative and thought-provoking, this is a significant new way of considering awards for failure of condition.