Until recently, the classroom assessment literature has emphasized the role of teachers and tests, for example investigating teachers' assessment practices or the quality of classroom tests and other assessments. In contrast, current understandings of teaching and learning emphasize the role of students, as well as the complex interactions between teachers, students, and contexts. We use the literature review method to give substance to a theory of classroom assessment as the co-regulation of learning by teachers, students, instructional materials, and contexts. We organize the literature using a version of Pintrich and Zusho's theory of the phases and areas of the self-regulation of learning, expanded to include the co-regulation of learning, in order to demonstrate how classroom assessment is related to all aspects of the regulation of learning. We conclude that this is a useful expansion for the field.