This research report from Brown University’s Annenberg Institute analyzes the racial-ethnic biases of teachers in math teaching. The authors explore expectations and judgments about student capabilities in math and teacher responsiveness to students’ mathematical thinking. Through simulated classroom scenarios composed predominantly of Black, Latinx/e, or White students, the authors find that teachers expected greater task difficulty in Black and Latinx/e classrooms compared to White students. They also find that teachers are more likely to support student sense-making and provide more positive affirmations to Black students, hinting at bias. Overall, this report has strong implications for teacher development and reforms in mathematics instruction. It would be particularly useful for teacher educator faculty who are preparing teachers to be informed of potential biases in their classroom