Early Childhood Development in Tonga offers a comprehensive assessment of early child development across Tonga using the Tongan Early Human Capability Index instrument. The data has information on more than 6,600 children, ages three to five, living across 36 inhabited islands, and reported for 129 communities. On the basis of population figures from the Tongan census data collection provided by the Tonga Department of Statistics, 81 percent of three- to five-year-olds participated in the Tongan Early Human Capability Index. The report details the development of the instrument used to collect the child development data to ensure cultural validity and local relevance, while still capturing the fundamental aspects of child development that are consistent across countries and cultures. As well as the development of the instrument, other countries will also be interested in learning about h the method of data collection across a country with remote and isolated islands using an innovative partnership between health and education. Using existing systems and community governance structures, the data was not only collected but also disseminated back to communities to raise awareness and prompt community and government mobilization to support early child development. The process of developing and implementing the Tongan Early Human Capability Index across Tonga helped build national and district capacity, and is encouraging the establishment of community-based supports for children. Researchers, policy makers, and practitioners as well as advocates for the development and enhancement of systems to monitor early child development worldwide will find this publication highly significant.