- Rule 13(a): A pleading must state as a counterclaim any claim that—at the time of its service—the pleader has against an opposing party of the claim: -- Arises out of the transaction or occurrence that is the subject matter of the opposing party's claim; and -- Does not require adding another party over whom the court cannot acquire jurisdiction - A counterclaim that arises out of the same transaction or occurrence - Supplemental jurisdiction will apply under 1367(a) - Preclusion may apply if fail to assert it