School Improvement Grants (SIGs) exemplify a capacity-building investment to spur sustainable changes in America's persistently lowest-performing schools and stimulate the economy. This study examines both short- and longer-term effects of the first two cohorts of SIG schools from four locations across the country. Dynamic difference-in-differences models show that SIGs' effects on achievement in Grades 3 to 8, as measured by state test scores in math and English language arts, gradually increased over the three reform years and were largely sustained for 3 or 4 years afterward. Evidence on high school graduation rates, though less robust, also suggests SIGs had positive effects. SIGs' effects on students of color and low-socioeconomic-status students were similar to or significantly larger than the overall effects.