Since the early 1970s, the Office of the Assistant Secretary for Planning and Evaluation (ASPE) has used the Transfer Income Model (TRIM) to assess how social welfare programs affect family incomes and poverty. TRIM began as a tool to analyze proposed reforms to the nation’s cash welfare program during the Nixon administration. It evolved into a uniquely comprehensive model of social welfare and tax programs as ASPE responded to new administration and congressional proposals. Today’s version of TRIM, called TRIM3, simulates over a dozen different programs--including cash assistance programs, nutrition benefits, other in-kind benefits, government-provided health insurance, payroll taxes, and federal and state income taxes and tax credits--and captures state-level policy variations as well as cross-program interactions. The model can be used in two ways: to examine how programs are currently affecting the economic well-being of American families, and to test what would happen (to program eligibility, program costs, tax liability, and so on) if policies were changed. This paper reviews the history and capabilities of TRIM3, and then details how the model has been used by ASPE to contribute to the policy debate over the past four decades. Past assessments of TRIM and possible future directions for the model are also discussed.