Opening Remarks to the Special Issue on Validation Theory and Research for a Population-Level Measure of Children’s Development, Wellbeing, and School Readiness Martin Guhn • Bruno D. Zumbo • Magdalena Janus • Clyde Hertzman Accepted: 1 March 2011 / Published online: 27 April 2011 Ó Springer Science+Business Media B.V. 2011 The articles in this special issue present an in-depth look at different theoretical, meth- odological, and empirical aspects of a comprehensive validation research program for a population-level measure of children’s developmental status and wellbeing. The measure that is at the centre of this special issue is the Early Development Instrument (EDI; Janus and Offord 2007; Guhn et al. 2007). The EDI is currently being used at a population-level in numerous Canadian cities and provinces, and as a national indicator of children’s wellbeing in Australia. The EDI is a measure that kindergarten teachers use to rate developmental outcomes of each child in their class on five domains: (1) physical health and wellbeing, (2) social competence, (3) emotional maturity, (4) language and cognitive development, and (5) communication skills and general knowledge. The primary purpose of the EDI is to provide communities with developmental information on all of the children in the community, at the developmentally important transition to grade school—and to thus provide communities with information they can use to discuss and inform practices, decisions, and policies that aim to foster the developmental wellbeing of children during the early years (cf. Janus and Offord 2007; Nosbush 2006; Guhn and Goelman, this issue). The special issue will appeal to a broad audience of practitioners, researchers, and policy makers interested in (1) research on children’s developmental health and wellbeing, (2) the construct of school readiness, and (3) validation of large-scale measurement ini- tiatives. The articles address multiple cutting-edge questions and challenges pertaining to a comprehensive, rigorous program of validation research for a measure that, in a general sense, may be considered a social indicator of children’s wellbeing at school entry. The articles may, of course, be of particular interest for those who use data from the EDI in M. Guhn Á B. D. Zumbo Á C. Hertzman University of British Columbia, Vancouver, BC, Canada M. Guhn (&) Human Early Learning Partnership, University of British Columbia, 4th Floor, Library Processing Centre, 2206 East Mall, Vancouver, BC V6T 1Z3, Canada e-mail: martin.guhn@ubc.ca M. Janus McMaster University, Hamilton, ON, Canada 123 Soc Indic Res (2011) 103:179–181 DOI 10.1007/s11205-011-9840-7