This paper provides a brief overview of the Urban Institute's Dynamic Simulation of Income Model (DYNASIM3). DYNASIM3 is a dynamic microsimulation model that projects the population and analyzes the long-run distributional consequences of retirement and aging issues. The model starts with a representative sample of individuals and families and ages the data year by year, simulating demographic and economic events including all key components of retirement income. Recent uses of DYNASIM3 include distributional analysis of Social Security reform options, the 2008 stock market crash, the Great Recession, and the impact of increased female labor force participation on retirement income.