This brief from the Learning Policy Institute synthesizes research on the classroom conditions that most effectively support K–12 mathematics learning, particularly for students from historically marginalized groups. Despite long-standing achievement gaps and pandemic-related setbacks, evidence shows that what happens inside classrooms can significantly improve outcomes. Drawing from the science of learning and development, the report highlights four key conditions that enable students to thrive in mathematics: positive teacher–student relationships, a strong sense of belonging, a growth mindset, and access to high-quality instruction. Together, these conditions create environments in which students feel safe, supported, and capable of success while engaging in challenging and meaningful mathematical work. The brief also outlines specific strategies teachers can adopt—such as positioning students as mathematically competent, reinforcing growth-oriented beliefs, and providing rigorous and culturally relevant learning opportunities. By attending to these conditions, educators can create equitable math classrooms that promote achievement and persistence for all learners.