This "Equity by Design" brief is intended for preK-12 school leaders who want to better understand the root causes of inequitable outcomes in their systems. Despite educators' often good intentions to effectively serve all students, there is ample evidence of ongoing disparities negatively impacting students of Color in U.S. schools. Whiteness -- defined as structural arrangements and ideologies of race dominance -- is a significant barrier to achieving educational equity in schools and districts, and Whiteness functions in every aspect of schooling. This brief is organized around these three topics: (1) what is Whiteness; (2) how does it function in schools; and (3) what can be done about it. The brief also provides reflective prompts that school leaders can use to engage in additional learning about Whiteness. There is a robust body of research and literature on this topic, so this brief distills that into a concise and practical guide for school leaders.