In Uganda and other low-resource settings, too few children are learning to read in the early grades. In response, countries have embarked on reading reform programs, often with external support. Unfortunately, rigorous evidence of these programs' impact remains limited. This article presents the causal impact on reading outcomes of the US Agency for International Development/School Health and Reading Program (SHRP), a mother tongue reading program that used clustered randomized controlled trials in 12 language communities. The findings suggest that significant reading gains are possible in complex, large-scale mother tongue reading programs, but their magnitude may vary by language. SHRP significantly positively affected literacy achievement in 9 of the 12 languages, an encouraging sign of such interventions' overall potential for impact. Language characteristics, including complexity, were more predictive of impact than implementation fidelity or socioeconomic factors. We recommend that literacy improvement program designs consider language complexity and characteristics, among other factors, to maximize impact.