Facing the economic and financial crises of our time, parties to a contract may be confronted with unforeseen circumstances. According to Dutch law, courts of law may modify the effects of their contract or they may set it aside, in whole or in part, upon the demand of one of the parties. Is this the solution to their problems? This question merits a strongly negative response.In the doctrine of unforeseen circumstances a balance is sought between two major principles. On the one hand, the principle that agreements are binding. On the other hand, the law has to be equally just in its consequences. Thus, the court should be able to deviate from the principle that contracts are binding, but not too soon and not too much.Where and how should the line be drawn? The 27th Leiden Yearbook of Private Law (BWKJ) is dedicated to this question. It covers general aspects of the influence of unforeseen circumstances, the application of the concept in various fields of law and comparative perspectives.