In this chapter, the author offers a rationale of comparative procedural law to understand the different relations nonparties may have with res judicata (in civil law) and collateral estoppel (in common law, mainly the United States). It shows how the civil law concept of res judicata—which traditionally precludes one party from requesting the same relief against the other—was adapted to bind nonparties, who in fact were considered parties for all relevant purposes. In contrast, the United States has for a while now allowed nonparties to assert issue preclusion against the party that already litigated, which is the outcome of a jurisprudential shift away from mutuality requirements in preclusion doctrine. Finally, this chapter argues for a transformation in the civil law concept of res judicata, as a general preclusion doctrine, to allow nonparties to assert it against defeated parties.