New Art. 25(5), added to Art. 25 (mutual agreement procedure) of the OECD Model as part of the 2008 Update, provides for the mandatory arbitration of unresolved issues arising in the course of a mutual agreement procedure. This article first examines the various factors that led to the adoption of Art. 25(5) and the stages of its development by the OECD. The article then discusses some technical aspects of the new arbitration provision, including the Sample Mutual Agreement setting out some procedural rules, which is attached as an Annex to Art. 25.