Tax treaty protection from international double taxation only goes as far as the treaty's substantive scope. Nations worldwide have adopted the text of Article 2 of the OECD Model Double Taxation Conventions (headed Taxes covered) in concluding bilateral treaties to prevent double taxation in the area of taxes on income and capital and taxes on estates, inheritances, and on gifts. The wording and structure of Article 2 give rise to a host of ambiguities, creating uncertainty for taxpayers regarding the taxes that come within treaty scope. A research strategy that draws on historic materials documenting the development of Article 2 throughout the League of Nations, OEEC, and OECD seeks to shed light on a provision that has retained its basic format and wording since the 1920s. Recent case law and academic literature are analyzed to gain a clearer picture of the common international concepts expressed in tax treaties that use the formulations proposed in the OECD Model Conventions. The research strategy, conceptual models, and proposed results aim to contribute to the understanding of the "taxes covered" and to guide subsequent research and heighten awareness of problems in the interpretation and application of the provision on substantive scope in tax treaties.(author's abstract)