In recent years, two research foci have garnered considerable interest in the mathematics education research community: 1) conceptualizing and measuring the unique and distinct knowledge mathematics teachers use in their practice, and 2) conceptualizing and exploring students’ mathematics identity formation and development. I seek to synthesize claims made across these two bodies of literature for the purpose of exploring the following question: In what ways is teachers’ knowledge of students’ mathematics identity formation and development a viable dimension of the knowledge mathematics teachers use in their practice? The exploration culminates in a working framework for teachers’ knowledge of students’ mathematics identity development and formation, and concludes with implications for mathematics teacher education.