The purposes of this paper are to highlight the foundations of multilevel construct validation, describe two methodological approaches and associated analytic techniques, and then apply these approaches and techniques to the multilevel construct validation of a widely-used school readiness measure called the Early Development Instrument (EDI; Janus and Offord 2007). Validation evidence is presented regarding the multilevel covariance structure of the EDI, the appropriateness of aggregation to classroom and neighbourhood levels, and the effects of teacher and classroom characteristics on these structural patterns. The results are then discussed in the context of the theoretical framework of the EDI, with suggestions for future validation work.