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. The Urban Institute has maintained, developed, and used the model under a series of contracts with ASPE. Today’s version of TRIM, TRIM3, simulates over a dozen benefit and tax programs, including detailed cross-program interactions. TRIM3 is used to understand how current programs affect families and to estimate the outcomes of policy changes. This paper reviews TRIM3’s capabilities and summarizes how TRIM3-based analysis has supported policy decisionmaking.