The Gas System Analysis Model (GSAM) is a modular, reservoir-based model of the North American natural gas market, developed by ICF Kaiser Inc., under the sponsorship of the U.S. Department of Energy. A major improvement of GSAM over previous models is its ability to build the gas supply curves from the ?bottom up?, based on its input database of over 16,000 gas reservoirs. This feature allows one to examine changes in technology or market factors and see their effect on individual reservoirs. In this way GSAM is free from assuming certain functional forms for the supply curves. These generated supply curves are then brought together with demand-side and market-based constraints into an integrating linear program that models the flows of gas in the North American pipeline network. The result is a set of equilibrium gas quantities and prices as well as the resultant flows. The approach used in GSAM provides a tool for policy and program analysis.