In this article I argue that by connecting assessment to learning and instruction and by enacting assessment as a collective practice we can see new opportunities for learning. The concept of mediated action and the notion of a collective zone of proximal development are used to theorize assessment. Two cases are included to support and make empirically visible the conceptual argument. The first case involves student teachers who assess exam papers in an online environment. The second case involves peer-group assessment among learners in an upper secondary school. Through the conceptual argument and the supporting cases assessment emerges as a practice involving process as well as product and a practice in which learning, teaching, and assessment are mutually constitutive of learners' development. Finally, some implications for practice are discussed, including some challenges related to the tension between individual and collective approaches. (Contains 1 table and 3 footnotes.)