This paper delineates general validity and research questions that are underlying an ongoing program of research pertaining to the Early Development Instrument (EDI, Janus and Offord 2007), a population-level measure, on which teachers rate kindergarten children's developmental outcomes in the social, emotional, physical, cognitive, and communicative domains. It describes the large-scale research projects that are using the EDI to measure children's developmental outcomes for entire populations (e.g., provinces, cities). Given the uniqueness of the EDI's use as monitoring tool, or social indicator, for children's developmental health at an early age, we spell out the opportunities and challenges that these projects provide with regard to validation research. The article sets the stage for the special issue, as it is dedicated to showcase the different aspects of the conceptual, theoretical, and empirical validation research as well as the collaborative community-based projects that are currently being undertaken with respect to the EDI.