There are occasions in civil litigation when, to the dismay of plaintiff's counsel, the statute of limitations has expired and it is ascertained that a necessary party defendant has not been brought into the litigation. Under certain circumstances, the doctrine of "relation back" allows the party to be joined, despite the expiration of the statute of limitations. This doctrine may apply where the necessary criteria, articulated in a three-pronged analysis, are satisfied to show that the new party is "united in interest" with a defendant over whom jurisdiction has been timely obtained. The doctrine of relation back has generated a substantial body of case law. It was recently re-examined at the end of 2023 by the Court of Appeals in Nemeth v. K-Tooling ,40 N.Y.3d 405 (2023), resulting in important clarification of ambiguities in the three-pronged analysis which have arisen over time.

The relation back doctrine is codified in CPLR §203, which provides that for purposes of computing the statute of limitations, a claim asserted in a complaint is interposed against the defendant, or a co-defendant united in interest with such defendant, when the action is commenced. Thus, where the relation back doctrine is found applicable, an action will be deemed timely commenced against a defendant not originally joined, notwithstanding the expiration of the statute of limitations, provided that this party is found to be "united in interest" with a defendant against whom the action was timely commenced.

Perhaps the seminal Court of Appeals language expressing the concept of "united in interest," was in Prudential Insurance Company of America v. Stone, 270 N.Y. 154, 159 (1936) where the court held: