This is an impact evaluation of the Technical Support to Failing Schools Program, a Chilean compensatory program that provided 4-year in-school technical assistance to low-performing schools to improve students' academic achievement. The author implemented a quasi-experimental design by using difference-in-differences estimation combined with propensity scores matching procedures to estimate treatment effects. The main findings were the following: (a) the program had positive effects on fourth-grade students' achievement in both language and mathematics; (b) program effect size was 0.23 standard deviations, and not sensitive to control for covariates; (c) there were larger effects for students in the middle part of the students' test-score distribution; (d) after the intervention had ceased, the program impact declined rapidly; and (e) the program reduced grade retention by 1.5 percentage points. (Contains 3 figures, 6 tables, and 17 notes.)