This is a guide to and commentary on the new procedural rules for arbitration adopted by the Permanent Court of Arbitration (PCA) in December 2012. The PCA is a unique arbitral institution - an intergovernmental organization counting over one hundred member states - with a rapidly growing annual caseload of arbitrations involving various combinations of states, state entities, intergovernmental organizations, and private parties. The 2012 PCA Rules are the most recent set of arbitral rules from any institution, and constitute a consolidation of four sets of PCA Rules drafted in the 1990s, and updated in light of PCA experience and the revision of other procedural regimes. They include special provisions adapted to arbitrations involving public entities and a number of novel provisions drafted on the basis of the PCA's experience administering arbitrations. In recent years, the PCA caseload has expanded to the extent that the total amount in dispute in PCA cases is estimated to be greater than that in any other arbitral institution, increasing the need for a comprehensive guide to arbitration under its auspices.