Assessment for learning (AfL) is integral to teaching and learning, and has as its central foci (i) pedagogical intervention in the immediacy of student learning, and (ii) the students' agency in the learning and assessment process. The role that students adopt in AfL is consistent with the idea of self-regulated learning, which involves students as metacognitively, motivationally, and behaviorally active agents in their own learning. Through an analysis of an extended sequence of classroom interaction for the purpose of obtaining evidence of learning, this paper demonstrates that self-regulation is supported through a temporary process of co-regulation between teacher and student in the context of AfL. Co-regulation is a construct derived from Vygotsky's concept of socially mediated learning, and the neo-Vygotskian perspective on human learning as a culturally based communicative process, through which knowledge is shared and constructed. Specific features of co-regulation illustrated through the analyses presented in the paper are (i) goal orientation, a focus on the learning to be achieved; (ii) scaffolding, the assistance the teacher provides to achieve a goal that is currently beyond students' unassisted efforts; (iii) intersubjectivity, a shared understanding based on a common focus of attention; (iv) the active construction of knowledge by students, rather than transference of knowledge from the teacher to the student; and (v) temporary support, provided through scaffolding and other external supports that students can ultimately appropriate as their own.