On June 16?17, 2015, the U.S. Department of Energy (DOE) convened the Optima Stakeholder Listening ay to share information on an emerging effort to concurrently investigate the optimization of fuels and vehicles. At the time of the stakeholder listening day, this effort was referred to as ?Optima.? The revised name of the effort is Co-Optimization of Fuels & Engines (Co-Optima). The effort and the listening day will use the term ?Co-Optima? hereafter in this report.Co-Optima officially began in FY 2016 and is a collaboration of the DOE Vehicle Technologies Office, Bioenergy Technologies Office, and the DOE national laboratories. In FY 2015, DOE invited industry stakeholders to the listening day to obtain critical input on the opportunities and challenges of this effort. The meeting was held in Golden, Colorado, and hosted by the National Renewable Energy Laboratory.