This paper predicts that the study of second language acquisition, as a young discipline of scientific inquiry in its own right, faces a bright future, but only if its scholarly community critically re-examines some notions and assumptions that have too long been taken for granted. First, it is time to reconsider familiar dichotomies, such as second versus foreign language and natural versus instructed language learning. Furthermore, it is worth checking whether and to what extent the puzzling phenomena to be explained by language acquisition theories do really exist (such as uniformity and success and fast acquisition rates in first language acquisition and universal developmental sequences in second language acquisition). The paper furthermore pleas for a multidisciplinary approach to the explanation of the fundamental puzzles of first and second language acquisition and bilingualism, including bridging the divide between psycholinguistic and socio-cultural theories.