The goal of this paper is to build on and consolidate previous work undertaken in preparation for the OECD's International Early Learning and Child Well-Being Study (IELS), providing an in-depth overview on the research base for the study. There are three specific aims of the paper: 1) to summarise research on associations between early learning and long-term outcomes; 2) to summarise research on the contexts that support early learning, and; 3) to use this research base to describe the need for better information to improve children's equitable early learning. The information presented in this paper has been shared in various ways with countries and partners throughout the process of planning for IELS. This working paper is intended to unite the empirical basis that led to the development of IELS, providing both participating countries and a wider audience with key insights about the research and principles underlying the study. (author abstract)