FEDSIM is an integrated model that consists of two programs: the Impact Module and the Projection Module. The Impact Module reads data representative of Federal offenders and "resentences" these offenders in conjunction with three major policy changes: sentencing guidelines, the Anti-Drug Abuse Act of 1986, and the career criminal provision of the 1984 Comprehensive Crime Control Act. This is the microsimulation component, which focuses on individual offenders. The output of this program consists of prison length-of-stay estimates for each offender, including a zero length of stay for probationers, the offender's supervision period, and the beginning and ending dates for all these episodes. The Projection Module reads the output from the Impact Module and performs population projections based on growth rates in the conviction populations. Following detailed descriptions of these modules, the authors contrast some of the projections they made with the model in mid-1987 and the actual changes that occurred through the end of 1991. Some results are presented for 1989 and 1990, depending on data availability. The paper concludes that the success of FEDSIM shows that by focusing on the sentencing component of the criminal justice process, the model predicted with remarkable accuracy changes in the probability of imprisonment, the time to be served by prisoners, and the growth in the prison population for at least 4 to 5 years into the future. The greatest error occurred in projecting future conviction rate trends for some offense categories. Model limitations and future directions are discussed. 26 references, 9 tables, and 12 figures