Governments have been examining the potential role of joint government interpretations of investment treaties at OECD-hosted intergovernmental investment roundtables. Now well-established in the model BITs and treaty practice of the NAFTA governments, express provisions for such joint interpretations have recently been included in an increasing range of treaties and investment policies around the world. But while a significant number of major recent treaties contain such express provisions, most investment treaties do not expressly address joint interpretations and thus leave the issue to more general rules. This paper addresses the general legal framework applicable to joint agreements by treaty parties about the interpretation of treaties. It outlines some key concepts and distinctions in treaty interpretation, and then considers the effects of treaty interpretations and amendments on third parties and in particular on investors covered by a treaty. Joint government interpretation can be binding or non-binding on investment arbitration tribunals. The paper concludes with brief consideration of possible criteria that could affect the persuasiveness of non-binding guidance.