Chapter 16 offers a brief overview of collateral estoppel in arbitration in the United States. It argues, initially, that, under Brazilian law, res judicata on final merits judgments and issue determination operate both ways between adjudication and arbitration, because the law gives the same effects to court judgments and to arbitration awards. It argues more generally that a system that provides for arbitration must enable this two-way function of preclusion, despite American courts having limited the estoppel effect of arbitration awards in many cases.