National and international studies such as the recent Survey of Adult Skills provide strong evidence of the need for and economic value of adult basic skills (ABS). Although ABS program evaluation and accountability reports typically show small gains for program participants in test scores and other outcomes, these studies rarely include comparison groups of nonparticipants, and most studies that do include such controls have not found statistically significant ABS program impact. The Longitudinal Study of Adult Learning (LSAL) is a lifelong and life-wide study. LSAL randomly sampled about 1,000 high school dropouts and followed them for nearly a decade from 1998-2007. LSAL followed both participants and nonparticipants in adult literacy programs, assessing their literacy skills and skill uses over long periods of time, along with changes in their social, educational, and economic status, offering a rich picture of adult literacy development. This is the second of a series of Research Briefs that utilize LSAL data to examine long-term impacts of ABS program participation on a range of outcome measures. This Research Brief addresses the following research questions: (1) What is the impact of participating in an ABS program on subsequent literacy proficiency? (2) What is the temporal course of that impact? and (3) To what extent does GED attainment mediate the impact of participation on proficiency?