Oscar Schachter was intensely concerned with establishing the basis for the obligation to obey international law. I trust he would forgive me for rendering my tribute to him in the form of an analysis of a concept that tends to undercut that obligation. That is the concept of rebus sic stantibus, more completely, clausula rebus sic stantibus. Developed legal systems have all struggled with the problems that arise when new developments unforeseen by the parties interfere with the performance of agreements as they were written. Municipal law systems use such terms as impossibility, frustration, imprévision, Wegfall der Geschäftsgrundlage, or the like to respond to these problems. The comparable issue has been mooted in international law for at least five centuries and has received a variety of answers. We investigate this problem first by examining the municipal law equivalents and working out the differences between the context of those national rules and that of the international legal system. We then examine the history of acceptance or rejection of the doctrine up to 1969 when the Vienna Convention on the Law of Treaties was formulated. We consider the development and the meaning of that Convention’s provision on rebus sic stantibus. Finally we gather the threads together in an attempt to see whether a prevailing rule can be identified and what the preferable rule would be.