Adult learners of a second language (L2) who have had little or no schooling in their first language do not seem to benefit as much from L2 instruction as higher educated L2 learners. Yet, we do not know in what ways L2 instruction can enhance the development of L2 skills in low print literate, low-educated L2 (LESLLA) learners. To date, there is very little research available on the effectiveness of instructed second language acquisition in this group of learners. The proportion of LESLLA learners is not small: around 40% of the immigrants in Belgium and other Western countries is low-literate and low-educated. This research project examines whether and how LESLLA learners benefit from L2 instruction by pursuing three goals: 1) determine to what extent learners with differing literacy and educational backgrounds gain from L2 instruction; 2) identify what teacher practices work best in L2 instruction to adult L2 learners, and LESLLA learners in particular; 3) explore how LESLLA learners process written L2 input. By gathering large-scale semi-longitudinal data, performing a teacher effectiveness study, and conducting an experimental study in a research population that has long been neglected in L2 research, this study will provide a rich picture on the effectiveness of current-day adult L2 instruction. This way, this study may yield far-reaching implications for both L2 research and instruction.