This paper describes the creation and intended uses of a longitudinal model of taxpayer behavior being developed by Treasury's Office of Tax Analysis. The model is based on a large new ten-year panel of individual taxpayers, their spouses and dependents. This paper describes Treasury's current (cross-sectional) methodolgy for distributing tax burdens by income, and how it could be enhanced by creating new longitudinal measures of income and taxes paid. We also describe the construction of the new panel dataset and how it could be extrapolated into future years to create an ongoing longitudinal model of taxpayer behavior.