The United States is a net importer of many important minerals, including tin. Consumption of primary tin in the United States is about 36,000 metric tons per year. Identified U.S. tin resources consist of about 40,000 metric tons. Although such figures provide insight about vulnerability to supply disruptions in the short term, they do not provide information about the potential of a country to meet its needs for a commodity from undiscovered domestic sources. Recent developments, including the preparation of models of mineral deposits and their grades and tonnages, and the application of computer simulation techniques to the estimation of metallic mineral resources, make it possible to estimate the magnitude of undiscovered resources, by deposit type, for relatively small areas such as the Seward Peninsula. This paper uses these developments and geophysical data to estimate undiscovered lode tin resources on the Seward Peninsula of Alaska. The assessment is based on a three-step methodology that (1) identifies the types of tin deposits that may be present in the region, (2) identifies the geophysical characteristics of unroofed granites and shallow granitoids, and (3) estimates, on the basis of various combinations of geologic and geophysical conditions, the number of undiscovered deposits present within or near exposed or concealed granite plutons. Computer simulation was used to combine the estimates of the number of deposits with available grade and tonnage models. Simulation experiments were designed to estimate the amount of tin in undiscovered deposits under a variety of limiting conditions. Results of simulation experiments indicate there is a 90 percent chance that the Seward Peninsula contains at least 51,000 metric tons, a 50 percent chance that it contains at least 390,000 metric tons, and a 10 percent chance it contains at least 1,100,000 metric tons of tin in undiscovered greisen, vein, and replacement deposits that have average grades of at least 0.5 percent tin.