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 Illinois, the legal concept of detrimental reliance is recognized and is often referred to as promissory estoppel. Under this doctrine, if one party makes a promise that the other party relies upon to their detriment, the promisor may be prevented from arguing that there was no enforceable contract. This typically occurs when there is no consideration, which is usually required to form a binding contract. Detrimental reliance can make an otherwise unenforceable promise enforceable if the reliance was reasonable, the promisee's reliance was foreseeable to the promisor, and the promisee suffered a detriment as a result of their reliance on the promise. This concept is rooted in principles of fairness and aims to prevent injustice that would result if the promisor were allowed to renege on their promise. It is important to note that detrimental reliance is not a separate cause of action in tort, but rather a principle applied in contract disputes to enforce promises that have been relied upon to the promisee's detriment.