This article examines the theory, history and operation of the question of "source of income" from the perspective of the taxation of intellectual property rights and of supplies of substantial intellectual content ("intellectual supplies") under tax treaties and its implications and possible evolution in the e-commerce context and the digital age. Specifically, the article discusses the theory of source taxation of intellectual property; intellectual supplies and tax treaties - the distinction between royalties and business profits; intellectual supplies in the digital age - operation of existing rules; and towards a coherent framework for the taxation of intellectual supplies in tax treaties. The article concludes that, although reform is presently premature, a new framework for taxing intellectual supplies will be required, and the article proposes a new framework for doing so.