This report considers the science of adult literacy acquisition, with the goal of identifying principles that may improve the effectiveness of adult literacy programs. We define literacy as a continuum, from emerging literacy, through improving literacy, to fluent literacy and we outline the factors that influence how far along that continuum an adult learner is likely to proceed. Four nested levels are identified at which obstacles to literacy progress operate, but to which adult learners may also bring strengths. We consider the evidence base for each of these levels: (1) "The Brain," where the basic neuroscience of how learning changes over the lifespan gives us clues about processes that restrict adult learning, and hints about how teaching can be optimized for the adult brain; (2) "The Person," where the roles of motivation, resilience and engagement can have a profound impact on outcomes; (3) "The Learning Environment," where teaching approach, course structure, fellow learners and technology can all affect progress; and (4) "The Cultural Context," where integrating expectations and local needs into literacy programs can allow learners to access materials in the most acceptable and appropriate ways. Principles derived from this evidence base are integrated and contextualized in an analysis of six case studies from the field. We indicate areas where further research is required (e.g., determining the degree of practice adults require to achieve fluency); and identify metrics that future literacy programs should collect in order to build an evidence base of properties that improve effectiveness. The evidence summarized in this report suggests that there is scope to improve outcomes in adult literacy programs around the world, and to realize more of the social and economic benefits that such gains provide. [This paper was commissioned for the World Bank activity on Innovations in Adult Literacy.]