The Zone of Proximal Development (ZPD), first proposed by the Russian psychologist L.S. Vygotsky, is frequently cited as the theoretical basis behind Dynamic Assessment (DA). However, this concept has led to varied interpretations and practices in education. This paper outlines readings of the ZPD that motivate many current approaches to DA and argues that the full potential of Vygotsky's proposal is realised through what he described as collective, transformative activity undertaken with learners. Through close analysis of transcribed interactions between a mediator and second language (L2) learners, we argue that this activity unfolds through the co-construction of collaborative and cooperative interactional frames. These allow for the integration of diagnosing and promoting learner development by alternately foregrounding the process of working through assessment tasks and going beyond those tasks to pursue other questions and problems. (Contains 3 notes.)