The leading authorities to know, and how the doctrine developed over time. Where one party to a contract makes a clear and unequivocal promise, intended to be acted on, that they will not enforce their strict legal rights, and the other party acts on that promise, equity may estop the promisor from going back on the promise to the extent it would be inequitable to do so.
The doctrinal seed, equity will not allow a party who has lulled another into a course of conduct to insist on strict rights.
Denning J's reformulation that gave the modern doctrine its name and its most-cited illustration.
Established that promissory estoppel is a shield, not a sword, it suspends rights but does not create a fresh cause of action.
Inequitable conduct by the promisee defeats reliance on estoppel, required for any complete answer on its limits.