Many local justice reform efforts are focusing on rethinking how jails are used and reducing jail populations and disparities. For jurisdictions to realize those goals, it is important that they understand what drives their jail populations, where to target interventions to address those dynamics, whether interventions are functioning as intended, and whether interventions are generating system impact. This case study, part of a series highlighting work supported by the Safety and Justice Challenge, examines how Charleston County, South Carolina and St. Louis County, Missouri use data to inform their work to reduce local jail populations.