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 Massachusetts, the doctrine of unjust enrichment is recognized and applied by courts when one party has received a benefit that would be unjust for them to retain without paying for it, and there is no valid contract governing the situation. This doctrine is based on principles of equity and restitution, rather than on any wrongdoing by the party who received the benefit. To establish a claim for unjust enrichment in Massachusetts, a plaintiff must demonstrate that they conferred a benefit upon the defendant, that the defendant appreciated or knew of the benefit, and that it would be inequitable for the defendant to retain the benefit without paying fair value for it. The goal of this doctrine is to prevent one party from being unjustly enriched at the expense of another, and to ensure that the party who provided the benefit is fairly compensated. The remedy typically involves the court ordering the enriched party to make restitution to the aggrieved party. This equitable relief is available when there is no adequate remedy at law, such as a breach of contract claim.