CIES 2018 Presentation, given by Rachel Jordan. In Uganda, and many other settings, too few children are learning how to read. In response, countries have embarked on national reading programs. Unfortunately, these programs are rarely evaluated rigorously at a large scale. This paper is based on larger research that efforts looked at the impact of a large-scale mother tongue reading program in Uganda using a randomized control trial in twelve language communities. It also looked at the differences in mother tongue reading acquisition attributed to linguistic differences, differences in program implementation and socioeconomic differences in the communities. The research points to language complexity as an important predictor of reading acquisition. Aspects of this complexity (transparency, tonal markings, agglutination) will be discussed in the 12 languages supported by current early grade reading reform efforts. Tying into language mapping efforts, the paper will also discuss the realities of language mapping on the ground where, for example, regardless of the predominant language spoken by the school community the school language of instruction is decided at the district level.