A legal principle that permits a party to appeal an interlocutory ruling immediately without waiting for a final determination of the underlying case. The collateral order doctrine is a narrow exception to the final judgment rule, which requires parties to wait for a final judgment before appealing any court ruling in a case (Midland Asphalt Corp. v. United States, 489 U.S. 794, 798 (1989)).

Generally, a final judgment is a decision by the court that terminates the litigation on the merits and leaves the lower court with nothing to do other than execute the judgment (Dandong v. Pinnacle Performance Ltd., , at *8 (S.D.N.Y. Dec. 12, 2011)). Under the collateral order doctrine, however, parties may appeal interlocutory rulings before a final judgment if the parties would lose their rights without an immediate appeal (Dandong, , at *8).

Under the collateral order doctrine, parties may appeal interlocutory rulings only if the order:

(Dandong, , at *8.)