The 20th Crosscut Lean/Low-temperature Exhaust Emissions Reduction Simulation (CLEERS) Workshop sponsored by the US Department of Energy and organized by the Oak Ridge National Laboratory was held on October 3?5, 2017 in Ann Arbor, Michigan. The workshop covers a wide range of emissions control research topics in gasoline, diesel, and natural gas combustion with an emphasis on simulations of modeling of their complex processes. A sample of the topics includes novel catalyst formulations, aging and poisoning, biofuel impacts, selective catalytic reduction of NOx, ultra-low NOx emissions, and perspectives toward 2025 emissions compliance. There were a total of 162 attendees representing automotive manufacturers, component and software suppliers, national labs, universities, and government agencies. There were 39 oral presentations including four invited speakers, 23 posters presentations, and a panel discussion on emerging low-temperature aftertreatment needs and modeling requirements. Many of the presentations have been posted online at http://www.cleers.org/. Some presentations, received as manuscripts, have been selected for publication in Emission Control Science and Technology as special articles from the 2017 CLEERS Workshop.