America’s 3,283 jails are the “front door” to mass incarceration. But for too long, county jail systems have operated and grown outside of public view. The Vera Institute of Justice's Incarceration Trends data tool, launched in 2015, illuminated the growth in local jail populations over the last 40 years. This report explores one of the Incarceration Trends project’s most startling revelations—that the main drivers of mass incarceration are small and rural counties, not major cities. Vera’s research identified two drivers of this trend: an increase in the number of people being held pretrial, and in the number of people being held for other authorities. Read the report to learn more, and explore the accompanying interactive data visualization that sheds light on the way that specific counties are using pretrial detention and rented beds.