When parties to a contract make promises to perform their obligations, and one party reasonably relies on the other party’s promise—but the party making the promise fails to perform, causing harm or loss to the party who relied on the promise—the party who relied on the promise to perform is said to have relied to its detriment.

This legal concept is called detrimental reliance. Detrimental reliance may serve as a substitute for consideration, and make an otherwise unenforceable contract enforceable.

Thus, detrimental reliance is a legal concept based on fairness (known as equity or equitable), and is equivalent to contractual promissory estoppel (due to the other party’s reliance, the party who did not keep its promise is prohibited from challenging the enforceability of its promise).

Detrimental reliance is not a separate tort cause of action.

In California, detrimental reliance, also known as promissory estoppel, is a doctrine that can make an otherwise unenforceable contract enforceable when one party relies on the promise of the other to their detriment. This concept is rooted in principles of fairness and equity. For promissory estoppel to apply, a party must have made a promise that the other party reasonably relied upon, and this reliance must have resulted in a detriment or loss. The reliance must have been foreseeable, and injustice can only be avoided by enforcing the promise. While detrimental reliance is not a separate tort, it is a legal remedy that prevents a party from arguing that their promise should not be upheld if the other party has suffered a loss due to their reliance on that promise. California courts will consider factors such as the nature and extent of the reliance, and the definiteness of the promise when determining whether to apply this doctrine.