This article concerns a particular application of Vygotsky's concept of the Zone of Proximal Development (ZPD) wherein conventional assessment situations are reorganized to allow for cooperation between assessor and learner as they jointly complete assessment tasks and work through difficulties that arise. This approach, known as Dynamic Assessment (DA), a term coined by Luria (1961), derives from Vygotsky's own work in the area of "defectology" and aims to reveal abilities that have fully developed as well as those that are still forming. Several models of DA have been developed and pursued in countries around the world, primarily in contexts of intelligence and abilities testing and in work with learners with special needs (Haywood & Lidz, 2007). We argue that DA in fact has profound implications not only for formal testing but for educational practice more generally, and for language education in particular, given that it posits a dialectical relation between instruction and assessment. Specifically, joint activity intended to reveal a learner's ZPD and the provision of mediation to support continued development are fully integrated in DA. Examples of this dialectical activity are presented involving classroom learners of French as a second language.