Despite a highly developed U.S. adult education system, 90% of adults aged 20 years and older considered the least educated did not participate recently in formal or nonformal education. What are nonparticipants’ characteristics, learning backgrounds, and skill levels? What predicts their likelihood of not participating in recent formal or nonformal education? The author analyzed 2012/2014 Program for the International Assessment of Adult Competencies–USA data. Situational deterrents of increasing age, parental education, low income, and work and family responsibilities contribute to nonparticipation. Dispositional deterrents include health and disability challenges, low social trust, and difficulties relating new ideas to real life. Institutional deterrents are education costs and little work schedule flexibility. Supports reported by nonparticipants are liking to learn new things, use of computers, and getting information from television and people they trust. Results from Program for the International Assessment of Adult Competencies–USA analyses inform adult and postsecondary educators and policy makers on what happened to—and how to reach—the forgotten 90%.