The doctrine of unjust enrichment applies the principles of restitution to disputes that are not governed by a contract between the parties. It characterizes the result of a failure to make restitution under circumstances that give rise to an implied or quasi-contractual obligation to return those benefits.

The courts describe this claim in general principles. For example, courts have stated that a claim for unjust enrichment seeks to restore money where equity and good conscience require restitution; it is not premised on wrongdoing, but seeks to determine to which party, in equity, justice, and law, the money belongs; and it seeks to prevent unconscionable loss to the payor and unjust enrichment to the payee.

Because recovery based on unjust enrichment of another party relies on the court's sense of fairness or equity rather than the law, it is often referred to as the equitable doctrine of unjust enrichment.

In Illinois, the doctrine of unjust enrichment is recognized and applied by courts when one party has been unjustly enriched at the expense of another, and there is no valid contract governing the transaction between them. This doctrine is rooted in principles of equity, aiming to prevent one party from retaining a benefit that in fairness belongs to another. When a claim for unjust enrichment is brought forth, the plaintiff must demonstrate that the defendant has received a benefit, and retention of that benefit without payment would be unjust. The remedy typically involves restitution, where the enriched party is required to compensate the party who suffered a loss. This remedy is not based on the existence of wrongdoing but is instead focused on the equitable distribution of benefits and burdens. Illinois courts will consider factors such as the relationship between the parties and the circumstances under which the benefit was received to determine whether unjust enrichment has occurred and what restitution is appropriate.