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 Missouri, the doctrine of unjust enrichment is recognized and applied by courts when one party has been enriched at the expense of another in circumstances that the law sees as unjust. This doctrine does not require a contractual relationship between the parties. Instead, it is based on the equitable principle that it is unfair for one party to retain benefits without paying for them when there is no legal justification for the retention. Missouri courts will consider an unjust enrichment claim to determine whether, in equity and good conscience, restitution should be made. The claim is aimed at preventing one party from suffering a loss while the other is unjustly enriched. The courts will look at the specific circumstances of each case to decide if an implied or quasi-contractual obligation exists, which would require the enriched party to make restitution to the other party. This is an equitable remedy, meaning it is based on principles of fairness rather than strict legal rules.