What is estoppel? The basic concept of an estoppel is that where a person (A) has caused another (B) to act on the basis of a particular state of affairs, A is prevented from going back on the words or conduct which led B to act on that basis, if certain conditions are satisfied. In such cases, A is estopped (ie 'stopped') from resiling from, or denying, the existence of that particular state of affairs. At the heart of all pleas of estoppel is the central question of whether it would be unjust for a party to resile from a position previously agreed with, or represented to, another party. Sometimes, the court will simply consider whether an estoppel has arisen, without necessarily specifying the type. In the context of construction projects, and as demonstrated by the examples in this Practice Note, the forms of estoppel most likely to be relevant are: Estoppel by representation An estoppel by representation arises where one person (A) makes, by words or conduct, a unilateral representation of fact or law to another (B), which was