Promissory estoppel is one of the broad categories of reliance-based estoppels. Promissory estoppel is differentiated from the other two forms of reliance-based estoppel, estoppel by representation of fact and proprietary estoppel, in that promissory estoppel applies where one person makes a promise to another person, but there is no contract that can be enforced to make the person carry out the promised action.
In order for promissory estoppel to apply, the party that has been victimized must prove in court that there was both an inducement and a detrimental reliance. In other words, there has to be evidence that one party intended for the victim to act on the promise or representation, or the victim must satisfy the court that their actions were a reasonable response to the relevant promise or representation.
The victim must also show that the actions that the victim engaged in were either reasonable or were the intended response to the representation made, and that the victim would suffer a loss or detriment at the current moment in the event the other party were permitted to be released from the assumed obligation. For the courts to find that promissory estoppel applies it must be shown that it would be unconscionable to allow the party to benefit from their actions.
Promissory estoppel and estoppel by representation of fact are mutually exclusive concepts. Estoppel by representation of fact is based on a representation of some mixture of law and fact, while promissory estoppel is based on a promise to fail to exercise a previously existing right.