Parallel litigation--a dispute generating multiple lawsuits--is not a new phenomenon and is not limited to celebrities or sensational controversies. Besides the publicity-generating lawsuits noted above, divorce actions have a long history of parallel lawsuits and conflicting judgments. Recent years have seen an expansion both in the incidence and the subject matter of parallel lawsuits, perhaps fueled by the traditional motivations of home-court advantage and differing laws, along with the expansion of personal jurisdiction rules in the past fifty years. In spite of this increase, the vocabulary remains imprecise and ambiguous. Parallel litigation would seem to mean identical or mirror image lawsuits between identical parties, but is often used when the lawsuits are not identical. Duplicative litigation has been defined as the "simultaneous prosecution of two or more suits in which some of the parties or issues are so closely related that the judgment in one will necessarily have a res judicata effect on the other." Earlier discussion have noted three categories of parallel litigation: (1) repetitive actions: multiple suits on the same claim by the same plaintiff against the same defendant; (2) reactive suits: a separate suit filed by a defendant in the first action against the plaintiff in the first action, seeking a declaratory judgment that he is not liable under the conditions of the first action or asserting an affirmative claim that arises out of the same transaction or occurrence as the first suit; and (3) separate actions by class members on the same cause of action raised in the class action, seeking to represent the same or a similar class. These categories are perfectly parallel and clearly subject to claim and issue preclusion, along with arguments that simultaneous prosecution is inefficient and wasteful. A distinct fourth category is "related litigation": separate suits involving similar parties or issues to which claim preclusion may not apply, but eligible for issue preclusion and to a lesser extent, subject to the same arguments as to wasteful litigation. Treatment here includes all four categories, with distinctions drawn as to their differing treatment in varying jurisdictions. In discussing these cases and their remedies, this Article will use the terms "parallel" and "duplicative" interchangeably, in reference both to identical and mirror image lawsuits, as well as substantially similar lawsuits with common questions of law or fact between substantially--but not always perfectly--identical parties. This Article discusses (1) repetitive suits by the same plaintiff against the same or similar defendants, (2) reactive suits filed by the defendant in the first action against the plaintiff in the first action, (3) declaratory judgment suits filed by a current or potential defendant lacking any real affirmative claim, and (4) separate actions by class members on the same cause of action raised in the class action.