A Practice Note explaining how US courts determine whether the court or the arbitral tribunal decides whether a claim is arbitrable. Specifically, this Note explains when judicial review of arbitrability is appropriate and how parties can delegate arbitrability to the arbitrator, including by delegating the decision to the arbitrator expressly in the arbitration agreement or by adopting institutional rules that delegate this authority. This Note also discusses the resolution of a circuit split on whether courts may conduct a limited threshold review to determine if the arbitrability claim is wholly groundless.