Global access to preschool has increased dramatically yet preschool quality is often poor. We use a randomized controlled trial to evaluate two approaches to improving the quality of Colombian preschools. We find that the first, which was rolled out nationwide and provides additional resources for materials and new staff, did not benefit children's development and, unintentionally, led teachers to reduce their involvement in classroom activities. The second approach additionally trains teachers to improve their pedagogical methods. We find this addition offset the negative effects on teacher behavior, improved the quality of teaching and raised children's cognition, language and school readiness. [This research was funded by the International Initiative for Impact Evaluation (3ie), Fundacíon Exito, European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 695300-HKADeC-ERC-2015-AdG), the ESRC Centre for Microeconomic Analysis of Public Policy at the Institute for Fiscal Studies, and the British Academy Visiting Fellowship VF10124.]