Outokumpu and its insurers sued GE Energy after motors supplied for an Alabama steel plant allegedly failed. The underlying contracts between the plant owner and the prime contractor required arbitration, but GE Energy was a nonsignatory subcontractor. The Supreme Court held that the New York Convention does not categorically prevent a nonsignatory from invoking domestic doctrines such as equitable estoppel to enforce an arbitration agreement. The Convention’s requirement that an arbitration agreement be in writing addresses the agreement itself, not the identity of every party who may enforce it under domestic law. The Court remanded for application of domestic law to determine whether GE Energy could enforce the clauses. Justice Sotomayor concurred, emphasizing that any nonsignatory doctrine must remain rooted in the Federal Arbitration Act’s foundational principle that arbitration is a matter of consent.