The Philippines Early Childhood Care and Development (ECCD) Longitudinal Study was a major research investment by UNICEF, the Philippines Department of Education, and the Australian Government. It was designed to investigate the impact of one of the most significant education reforms in Philippines history: the introduction of the Kindergarten year as part of the K-12 Basic Education System. It offers useful insights into the impact of Philippines Mother Tongue-Based Multilingual Education (MTB-MLE) and key language transition points for students from Kindergarten to Grade 4. This longitudinal study measured not only the impact of Kindergarten, but also the contribution that preschool education made to student achievement and growth across the first five years of school.