The authors point to systemic-theoretical instruction (STI), which underscores the importance of abstract conceptual knowledge in schooling, and dynamic assessment (DA), in which mediators and learners function cooperatively, as examples of the theory-practice relation envisioned by Vygotsky (1987). This article proposes an interactional framework to bridge STI and DA. Mediated development (MD) emphasizes mediator-learner dialogic interaction in which the aim is not to assess emerging capabilities but rather to guide learner appropriation of concept-based instructional materials as symbolic tools for thinking. The researchers situate MD in relation to the broader Vygotskian second language research tradition, while also acknowledging the contributions of Israeli researcher Reuven Feuerstein (Feuerstein, Feuerstein, & Falik, 2010). Feuerstein's work to promote general cognitive abilities among learners with special needs closely parallels Vygotsky's research and was influential in developing this proposal of MD. The authors discuss principles of the framework and illustrate them through close analysis of excerpts of mediator-learner interactions from an STI program to teach the English tense-aspect system in a university-level ESL academic writing program. Analysis reveals the value of mediator guidance in helping learners to recognize the relevance of instructional materials and to begin to use them as tools during communicative activity.