A common problem in contract law occurs when a supervening change of circumstances impacts upon contractual relations. Although performance does not become impossible, it may become more difficult. Indeed, the whole basis on which consent was originally based may fall away. In medieval times the impact of changed circumstances on a contract was mitigated by an implied condition, which reserved the right of a contracting party to withdraw from that contract if he or she would never have promised with hindsight of the altered circumstances. This doctrine came to be known as the clausula rebus sic stantibus. This article will trace the life of the clausula doctrine, from its origins in Roman moral philosophy, through canon law and the writings of theologians, to its arrival as a fully fledged doctrine in civil law. While change of circumstances remains a problem in contract today and is recognised as a ground for voiding – or even intervening in – contracts in some jurisdictions, it is not recognised in others, such as South Africa. This is due largely to a concern for certainty in contract, which caused the demise of the clausula doctrine in later years. This article will end with the fall from favour of this concept in the age of codification.