The authors describe an evolving theoretical framework that has been called one of the best kept secrets of academia: cultural-historical activity theory, the result of proposals Lev Vygotsky first articulated but that his students and followers substantially developed to constitute much expanded forms in its second and third generations. Besides showing that activity theory transforms how research should proceed regarding language, language learning, and literacy in particular, the authors demonstrate how it is a theory for praxis, thereby offering the potential to overcome some of the most profound problems that have plagued both educational theorizing and practice.