One in six American adults--approximately 30-35 million individuals--were at one time enrolled in college but did not complete a degree or certificate. Higher education in the 21st century faces a series of changes with respect to college completion that require a shift in institutional culture and processes. Many colleges are responding to this changing environment and heeding the call to better support students in completing their degrees. This report is the culmination of four years of literature review, site visits, interviews, focus groups, and surveys of grantees conducted as a part of Higher Ed Insight's evaluation of Lumina Foundation's adult college completion efforts. The focus of the evaluation was to highlight critical early lessons that could be learned from Lumina-funded and related projects about the services and supports adults with some college credit need if they are to return to college and successfully complete a degree or credential. This article draws from the lessons learned from this evaluation as well as those identified by the many organizations working toward greater success for adults with some college credit. [This article is an abbreviated version of the full report, "Adult College Completion in the 21st Century: What We Know and What We Don't," which can be found on the Higher Ed Insight website: www.higheredinsight.com.]