Sections 1 and 2 of this treatise begin with a brief description of the OECD Model and its commentaries. In section 3 the application of art. 31 and art. 32 of the Vienna Convention on the Law of Treaties to the commentaries is analysed. Section 4 examines the extent to which the commentaries constitute binding rules of international law and section 5 discusses the effect of observations entered by OECD member countries on the commentaries. Section 6 discusses the effect on tax treaties of additions and changes to the commentaries published after the time such treaties have been concluded. Lastly, section 7 summarizes the conclusions reached in the previous sections.