For some Western European legal systems the principle of good faith has proved central to the development of their law of contracts, while in others it has been marginalised or even rejected. This book starts by surveying the use or neglect of good faith in these legal systems and explaining its historical origins. The central part of the book takes thirty situations which would, in some legal systems, attract the application of good faith, analyses them according to fifteen national legal systems and assesses the practical significance of both the principle of good faith and its relationship to other contractual and non-contractual doctrines and forms of regulation in each situation. The book concludes by explaining how European lawyers, whether from a civil or common law background, may need to come to terms with the principle of good faith. This is the first completed project of The Common Core of European Private Law launched at the University of Trento.