Res judicata is hard enough already. Consider it at the interjurisdictional level, and we are asking for headaches. But consider it at that level we must, because litigation trends make interjurisdictional preclusion more important than ever. Lawyers, judges, litigants, and other litigation participants increasingly must contemplate the possibility that a lawsuit will have claim-preclusive or issue-preclusive effect in a subsequent suit in another jurisdiction. With great frequency, multiple lawsuits arise out of single or related transactions or events. Mass tort litigation and complex commercial litigation provide the most emphatic examples, but the phenomenon of multiple related lawsuits extends to every comer of litigation, including intellectual property, matrimonial, criminal, antitrust, personal injury, securities, commercial, products liability, environmental, and civil rights. The participants in those multiple related lawsuits, moreover, increasingly find themselves litigating in far-flung forums. With continuing advances in transportation and communication, growing nationalization and internationalization of commerce, and an increasingly mobile society resulting in geographically dispersed family and business networks, it is hardly surprising that related lawsuits crop up in multiple jurisdictions. Furthermore, related lawsuits frequently are filed in both state and federal courts. Thus, controversies become multijurisdictional across two dimensions – “horizontally” among different state courts and “vertically” between state and federal courts.