If K-12 students are to be fully integrated as active participants in their own learning, understanding how they interpret formative assessment feedback is needed. The objective of this article is to advance three claims about why teachers and assessment scholars/specialists may have little understanding of students' interpretation of formative assessment feedback. The three claims are as follows. First, there is little systematic research of K-12 students' interpretations of feedback. Systematic research requires gathering substantive evidence of students' cognitive and emotional processes using psychological methods and tools. Second, there is an overemphasis on the external assessment process at the expense of uncovering learners' internal reasoning and emotional processes. This overemphasis may be due to vestiges of behavioral approaches and lack of training in social cognitive methods. Third, there are psychological tools such as the clinical interview, pioneered by Piaget and used by psychologists to "enter the child's mind," which may be helpful in uncovering students' interpretation of feedback and associated behavioral responses. If the purpose of formative assessment is to change student learning, and feedback is delivered as a conduit to help with this long-term change, understanding students' interpretation of feedback plays a central role in the validity of the process.