One of the enduring problems in education is the persistence of achievement gaps between White, wealthy, native English-speaking students and their counterparts who are minority, lower-income, or English language learners. This study shows that one intensive technical assistance (TA) intervention--California's District Assistance and Intervention Teams (DAITs)--implemented in conjunction with a high-stakes accountability policy improves the math and English performance of traditionally underserved students. Using a 6-year panel of student-level data from California, we find that the DAIT intervention significantly reduces achievement gaps between Black, Hispanic, and poor students and their White and wealthier peers. These results indicate that capacity-building TA helps to close achievement gaps in California's lowest performing districts.