1. Promissory estoppel
a) Background: The rule that consideration must not be past is sometime inadequate when a fresh promise is made and the other party acts upon it, but this act does not amount to (present) consideration in respect of this promise because nothing moves to the promissor, and the promissee suffers loss in reliance upon the promise when the promisor fails to act up to it.
b) Solution: doctrine of promissory estoppel: somebody who has made a clear and unambiguous promise cannot resile from that promise. Rules:
aa) The promissor must have intentionally modified existing contractual rights: Central London Property Trust Ltd. v. High Trees House Ltd. 1949 - also the leading case.
However, although obiter, it has been expressed that the doctrine can not only be invoked when existing contractual rights have been promised to be modified, but also when a contract is promised: i.e. A promises a job to B, who then takes it up.
bb) The person sued must have acted in reliance upon the promise made: D & C Builders Ltd. v. Rees 1966
cc) The doctine is a shield, not a sword; it can be invoked to avert a claim or to rebut a defence that might otherwise not be available; it cannot be used to found a claim: Combe v. Combe 1951
dd) The doctrine is suspensive, not permanent. So a promise can be made to suspend one's right for a limited period of time.

2. Proprietary estoppel
a) Where one person leads another to suppose that he is granting, or will grant, the other rights over his property (mostly land) and the other acts to his detriment in reliance upon the real or supposed grant, the doctrine of proprietary estoppel can be invoked to make up for the defects of the law governing consideration.
Note that this is confined to cases where one is encouraged to use land (or, possibly, other property).
b) In contrast to promissory estoppel, this doctrine can be applied only when the person acting on the promise has acted not only in reliance of it, but also to his detriment: Inwards v. Baker 1965
c) In contrast to promissory estoppel, the reliance may depend upon a promise which is merely implied; it need not be 'clear and unambiguous'.
d) In contrast to promissory estoppel, the plaintiff can support a claim on the basis of the defendant's behaviour:
Crabb v. Arun District Council 1976

3. Notabene
a) Estoppel is primarily a rule of evidence.
b) It means refusing to permit a person who has made a promise, or acquiesced in a state of affairs, to go back on his promise or withdraw his acquiescence.