With growing public attention to the problem of mass incarceration, people want to know about women’s experience with incarceration. How many women are held in prisons, jails, and other correctional facilities in the United States? And why are they there? How is their experience different from men’s? While these are important questions, finding those answers requires not only disentangling the country’s decentralized and overlapping criminal justice systems, but also unearthing the frustratingly hard to find and often altogether missing data on gender. This report attempts to do just that.