The United States has earned its nickname as a mass incarceration nation. The federal criminal justice system has contributed to this status with its own increasing rate of incarceration. The federal system now ranks as the largest population of sentenced prisoners in the country; it is even larger than the national prisoner populations among all European countries, save one. This is a recent phenomenon. This Article ties the increase in the federal incarceration rate to policies adopted by the U.S. Sentencing Commission since its inception that presume imprisonment as the default sentence. Since the Sentencing Commission’s creation in 1984, the proportion of federal sentences requiring incarceration increased from under 50% to over 90%. This Article provides evidence that the prison-by-default position by the Sentencing Commission is contrary to congressional intent when the Legislature passed sentencing reform laws in the 1980s, has contributed to a federal prison system that is operating over capacity, and wastes resources. The increasing rate of imprisonment at the federal level conflicts with the downward trend in national crime rates and with the states’ sentencing experiences in which probation sentences continue to be preferred. Potential alternative explanations for the significant trend toward the affirmative use of imprisonment in federal sentences are outlined, yet the available statistical evidence generally rules them out. Finally, suggestions on changes to the sentencing guidelines and to judicial sentencing practices are offered.