This article describes the recent and continuing efforts by the states of the United States to reform and harmonize their retail sales taxes under the auspices of the Streamlined Sales Tax Project. Although many aspects of the Project are of only marginal interest outside of the United States, some aspects of the Project address issues of broader concern. This article focuses on those issues and, in particular, on the development of general principles for assigning or "sourcing" the retail sales tax base in connection with cross-border transactions and on the Project's enlistment of third-party intermediaries to facilitate tax collection. Specifically, the article provides an overview of the US state retail sales tax regime and of the Project, including the adoption of the Streamlined Sales and Use Tax Agreement. The article also examines the sourcing provisions of the Agreement and its provisions regarding certified tax collection intermediaries and software.