Contractual estoppel has been developed in the context of the exclusion of liability for misrepresentation. It provides a legal explanation for the validity of ‘no representation’ and ‘no reliance’ clauses, which may contradict the true state of affairs and prevent a claim for misrepresentation arising. The importance of contractual estoppel does not end there for it may be applied more generally to prevent parties denying the existence of a state of affairs which was the basis of their contract. This paper seeks to answer two central questions that continue to trouble the courts, most commonly when a claim is based upon the alleged mis-selling of a financial product. The questions are: (1) What is the true nature of contractual estoppel? (2) Are ‘no representation’ and ‘no reliance’ clauses subject to the test of reasonableness set out in the Unfair Contract Terms Act 1977, as extended to contractual terms which ‘exclude or restrict’ liability for misrepresentation by s 3 of the Misrepresentation Act 1967?