This article focuses on the core US source rules and particularly the first of the functions that has been assigned to such rules - their contribution to the assertion of US authority to impose tax on foreign persons. The purpose of the article is to explore the current state of inbound "source rule law" in the United States and to prepare the ground for an inquiry whether the fast-developing commerce and technology of the 21st century call for a rethinking. The article concludes that it would be unrealistic (and probably unnecessary) to contemplate important changes in the US source rules in order to accommodate innovations in technology and commerce, particularly with respect to e-commerce and new financial concepts, tools and products, and that, in any event, the issues raised by "21st century" developments lie more in the area of income characterization than income source.