The purpose of this study was to understand ways in which teachers define student self-assessment, explore teachers' reasons for using or not using student self-assessment, and explicate the beliefs associated with these reasons. Drawing on Ajzen's theory of planned behavior, the authors interviewed seven high school teachers and explored the relationships between their definitions, beliefs, and reasons for using or not using student self-assessment. Definitions of self-assessment varied between formative and summative. Beliefs associated with student outcomes were indicators for using self-assessment. Beliefs about resources (e.g. time, information) and concerns about students' ability to self-assess were associated with not using self-assessment. The study has relevance for understanding ways in which efforts to increase use of student self-assessment likely depend on individual teacher definitions, reasons, and beliefs.