There is not yet a globally-accepted definition of ‘developmentally on track’. The Multiple Indicator Cluster Survey’s Early Childhood Development Index (ECDI) presently defines ‘on track’ as the percentage of children aged 36-59 months who are developmentally on-track in at least three of the following four domains: literacy-numeracy, physical, socio-emotional and learning. Other measures use different definitions, with varying empirically- and conceptually-driven perspectives on how best to define ‘on track’. Methodological work is ongoing to define commonly agreed terms and methods for more comprehensively assessing each of the domains of early childhood development (i.e. health, learning and psychosocial well-being).