The Illinois Basin – Decatur Project (IBDP) is a one million tonne deep saline geologic carbon dioxide (CO₂) storage project led by the Midwest Geologic Sequestration Consortium (MGSC), one of the US Department of Energy – National Energy Technology Laboratory’s Regional Sequestration Partnerships. The MGSC, lead by the Illinois, Indiana, and Kentucky Geological Surveys, was funded in 2003 to conduct geologic sequestration assessments and demonstrations throughout the Illinois Basin. The IBDP began in 2007 with a three-year pre-injection period, followed by three years of injection, and 3 years post-injeciton monitoring. Injection was successfully completed in November 2014 with over 999,000 tonnes of CO₂ injected at a nominal rate of 1,000 tonnes/day into the Mt. Simon Sandstone. The injectivity and capacity of the Mt. Simon have been confirmed. Cased-hole logging shows that CO₂ volumes injected remain in the lower Mt. Simon. Reservoir simulation shows the sealing capacity of the Mt. Simon-Eau Claire Shale interface will not be reached more than 100 years after the cease of injection. The integrated compression/dehydration, pipeline, and injection well system operated as planned with Archer Daniels Midland carrying out 24/7 injection operations fully integrated with their ethanol production facility. Intensive environmental monitoring of the near-surface environment above the CO₂ plume shows no effects on air, water, soil, or structural uplift during injection and none are expected. IBDP has demonstrated that the 1 million tonne volume is essential to address geophysical plume detection, modeling of an Area of Review, deployment of downhole data collection systems suitable to meet regulatory requirements, and to monitor and verify microseismic events as tied to pressure perturbations. IBDP is currently in the post-injection monitoring phase and is linked to the Illinois Industrial Sources CCS Project through scientific and permitting-related activities. These projects hold the first-ever Underground Injection Control permits for Class VI, specifically developed for the subsurface storage of CO₂, in the United States. Lessons learned from the first eight years of the Illinois Basin – Decatur Project will be discussed.