This paper puts forward a research agenda in the area of explicit and implicit knowledge and learning of second or additional languages. Based on a brief overview of reliable findings as well as open questions in the field, three agenda items are highlighted. First, valid and reliable measures of explicit and, in particular, implicit knowledge and learning need to be identified and their suitability for participants of different ages established. Second, and closely related to the previous point, explicit and implicit knowledge and learning should be investigated across the human lifespan. Therefore, studies need to include to a greater extent hitherto under-represented groups such as children and older adults in order to pinpoint the benefits or otherwise of implicit and, in particular, explicit knowledge and learning in these age ranges. Third, researchers should aim to capture with their designs the complex and dynamic interplay of the multiple cognitive, affective, biographical and contextual factors that influence the development of explicit and implicit knowledge over time. Concrete tasks for future research are proposed under these three agenda items, with a view to assisting interested investigators in formulating research questions and specifying research designs.