One of the eight Millennium Development Goals is that all children in developing countries should complete primary education. Much progress has been made toward this goal, but completing primary school does not ensure that students attain basic literacy and numeracy skills. Indeed, there is ample evidence that many children in developing countries are not learning these basic skills. This raises the question: What can schools and communities do to increase the learning that takes place in schools? Sri Lanka exemplifies these issues. It has achieved universal primary completion, but many Sri Lankan primary school students perform poorly on academic tests. This paper uses unusually rich data from Sri Lanka to investigate the determinants of academic performance, as measured by achievement tests, of Grade 4 students. At the child and household level, educated parents, better nutrition, high daily attendance, enrollment in private tutoring classes, exercise books, electric lighting, and children's books at home all appear to increase learning, while hearing problems have a strong negative effect. Among school variables, principals' and teachers' years of experience, collaborating with other schools in a "school family," and meetings between parents and teachers all appear to have positive impacts on students' scores. Estimates that exclude some of the variables available in the unusually rich data yield different results, which suggests that results based on less complete data are likely to suffer from omitted variable bias. A final section provides recommendations for education policies in Sri Lanka. (Contains 10 tables and 21 notes.)