Student self-assessment is a central component of current conceptions of formative and classroom assessment. The research on self-assessment has focused on its efficacy in promoting both academic achievement and self-regulated learning, with little concern for issues of validity. Because reliability of testing is considered a sine qua non for the validity of assessment interpretations, and research into the human ability to self-evaluate work raises concerns about the quality of students' judgements, it is sensible to investigate the accuracy of students' self-assessments. This article reviews relevant literature from educational psychology and psychometrics to define the need for a better understanding of accuracy in self-assessment as well as to identify possible pitfalls in measuring accuracy that could undermine its effectiveness by, for example, trading the focus on formative feedback for summative scoring or rating. The article concludes with recommendations for the design of research on accuracy in self-assessment.