Reliance on the promises and assertions of others, express or implied, is very common in human affairs, and its protection has been a prominent feature of Anglo-American law. Effective protection has often been afforded by contract law, but in a number of circumstances reliance occurs in the absence of an enforceable contract, and here other legal concepts have been deployed. But it has not proved possible to explain the protection of reliance in terms of any other single legal concept.

Very frequently one person makes an assertion on which another relies. In a wide variety of circumstances the courts have held that the person making the assertion is precluded from later contradicting it. The clearest examples relate to statements of fact. If a deadline for enrolment in a university programme is imminent, and a prospective student is informed by the university, before the deadline and while spaces remain in the programme, that she is duly enrolled, the university is estopped (i.e., precluded), after the deadline has passed, from denying the truth of the information, even if the student in fact was not duly enrolled, and even in the face of a mandatory rule in the most stringent language imaginable to the effect that no student shall in any circumstances be enrolled after the deadline. It will be seen from this example that the principle of estoppel is capable of having far-reaching effects on other legal rules, and it has often been the source of obligations that are not assignable to any of the usual divisions shown on diagrams or maps.