This study examines the relationship between teachers’ beliefs and instructional practices related to learning, pedagogy, and mathematics in regards to components of authenticity for two high-school mathematics teachers. Research on mathematics teachers’ beliefs has shown a relationship between teachers’ beliefs about learning, pedagogy, and mathematics and their pedagogical practices. The findings in this study are consistent with this literature revealing that individual teacher beliefs about authenticity are related to the teacher’s pedagogical practices. By using interview and observational data, beliefs are shown to be highly associated with instruction. The more a teacher’s beliefs were rated authentic, the more his or her instruction contained components of authenticity. Furthermore, teachers’ beliefs about Construction of Knowledge was the best indicator of authentic practices.