This document summarizes a study that examines the effect of medium of instruction on learning outcomes using longitudinal data from the Young Lives study in Andhra Pradesh, India. The study finds that students who receive instruction in their mother tongue (Telugu) perform significantly better on math tests on average compared to students who receive English-medium instruction, after controlling for student ability, household characteristics, and parental aspirations. This suggests that introducing English as the medium of instruction at early grades may negatively impact student learning outcomes. The study uses a value-added model and data from over 2,000 students in the younger cohort of the Young Lives study to analyze differences in math achievement scores between Telugu and English medium students at primary school level in India.