Rule 17.Parties Plaintiff and Defendant; Capacity.

Rule 17 requires every action to be prosecuted in the name of the real party in interest, the person who holds the right being sued on, though an executor, administrator, guardian, bailee, trustee of an express trust, a contracting party suing for another's benefit, or someone authorized by statute may sue in their own name without joining the person they represent. A case isn't dismissed for suing in the wrong name until the real party in interest has had a reasonable chance to ratify, join, or be substituted into the action, and doing so counts as if the suit had been properly brought from the start. An infant or incompetent person may sue or defend through a guardian, or, lacking one, through a next friend or a court-appointed guardian ad litem.

Rule 17(d) also lets a plaintiff who can't identify a defendant despite a diligent, good-faith effort name that defendant fictitiously, describing what is known about the person's interest in the case and the efforts already made to learn their identity. The unidentified defendant is treated as a party with notice of the suit from the start, and the plaintiff may later move to substitute the defendant's real name once it becomes known, supported by an affidavit of diligence; the court can protect the newly named defendant from undue burden, or dismiss claims against a defendant identified too late if the delay caused substantial prejudice and justice requires it.