This article proposes a model of formative assessment grounded in Vygotsky's theory of concept formation and argues that this model can provide a useful framework for facilitating a beginning teacher's continued learning. The model is used to argue that beginning teachers need to know how to recognize, describe, and use students' prior knowledge not only in terms of whether students get the academic concept but also in terms of the valuable, experience-based aspects of what students do know. The author demonstrates the model's utility by describing the results of a 3-year classroom research study on preservice teachers' conceptions of students' prior knowledge and formative assessment. A "get it or don't" conception was commonly used by preservice teachers and was found to have serious impacts on their instructional practices. The article concludes by exploring the potential of a theory-enhanced model of formative assessment for teacher educators' own instructional practices. (Contains 1 figure and 7 notes.)