Periodically, ISIS updates its estimates of global inventories of nuclear explosive materials. The most detailed description of nuclear explosive material inventories remains Plutonium and Highly Enriched Uranium 1996: World Inventories, Capabilities, and Policies.[1] Previous updates to that study were presented in the ISIS report The Challenges of Fissile Material Control and at various locations on the ISIS web site. This update is a set of reports, tables, and charts that detail inventories of nuclear explosive materials throughout the world. With this update, the inventories have been expanded to include estimates of neptunium 237 and americium holdings by country. This update also includes the new report "Civil HEU Watch," which breaks down civil HEU holdings by country, and a study of China's military plutonium and HEU holdings. Resources limited ISIS's ability to compile this information into a book or comprehensive report. The estimates in this study are given as a value as of the end of the 2003 calendar year. Some reports also present projections of likely future stocks. Uncertainties are a key part of these estimates. In summary tables or figures, these uncertainties are omitted, but the uncertainties remain an integral part of the estimate. Uncertainties, which in some cases are large, reflect the limited state of knowledge of some stocks, something a central estimate cannot do. ISIS is continuing to refine its estimates and produce new assessments. These findings are expected to be added once they are completed. This update replaces a similar study produced by ISIS in 2004. Links to the previous study have been removed from ISIS's homepage, but it can be accessed by clicking here. ISIS welcomes comments about this update and earlier work. [1] David Albright, Frans Berkhout and William Walker, Plutonium and Highly Enriched Uranium 1996: World Inventories, Capabilities, and Policies (Oxford: Stockholm International Peace Research Institute [SIPRI] and Oxford University Press, 1997).