Behind the learning crisis in much of the developing world is a huge data gap. Only a few middle income developing countries have the political incentives and technical capacity to develop and sustain national systems that measure what children are learning in school; most school children in the developing world have never taken a test that can be compared year over year or globally benchmarked. As enrollment has increased rapidly over the last two decades, policymakers and citizens have had no basis to assess whether more schooling has led to more learning, or respond with reforms and adjustments to improve education systems. This paper sets out five concrete recommendations, addressed to the international community, which together could go a long way toward filling the global data gap on learning outcomes in the next decade. These recommendations constitute an ambitious agenda for developing countries and the broader development community—but one that is entirely affordable, costing only about $400 million over the next 10-15 years.