The Knowledge Is Power Program (KIPP) is an influential and rapidly growing nationwide network of charter schools serving primarily disadvantaged minority students. Prominent elements of KIPP's educational model include high expectations for student achievement and behavior, and a substantial increase in time in school. KIPP is being watched closely by policy makers and educators as a possible model for urban education, but existing studies of KIPP's effects on students have been subject to methodological limitations, making them less than conclusive. We measure the achievement impacts of forty-one KIPP middle schools across the country, using propensity-score matching to identify traditional public school students with similar characteristics and prior-achievement histories as students who enter KIPP. We find consistently positive and statistically significant impacts of KIPP on student achievement, with larger impacts in math than reading. These impacts persist over four years following admission, and are not driven by attrition of low performers from KIPP schools.