The World Bank is committed to ending extreme poverty and building shared prosperity and ensuring that children are healthy and ready to learn is a critical part of this. Recently, researchers worked with the Government of Cambodia to evaluate the impact of three pilot early childhood development programs that were being scaled up with assistance from the World Bank. The evaluation found that expanding programs can lead to new challenges that might not be apparent when working on smaller pilots. In this case, unforeseen implementation problems--some related to delays in building schools or problems paying teachers--limited the program's reach and effectiveness. Parents who worked all day, or worked far from the village center, also found the part-time hours of the preschools difficult to manage, contributing to low use of the preschool services. The results of the evaluation highlight challenges governments can face as they move from pilot to scale up. (author abstract)