This Practice Note discusses the consequences of the doctrine of separability of the arbitration clause and considers how a court or tribunal, in England and Wales, determines the applicable law of the arbitration agreement in the absence of an express choice, whether in the clause itself or by incorporation of arbitration rules, and also considers why this can be important in (international) arbitration proceedings. This topic may be referred to as establishing the governing law of the arbitration agreement, determining the law applicable to an arbitration clause, the importance or significance of the law of the arbitration agreement, the proper law of the arbitration agreement, the principles for establishing or ascertaining the applicable law of the arbitration agreement.