The primary purpose of this Toolkit is to provide a resource for researchers, evaluators, and program personnel from various disciplines interested in assessing early childhood development (ECD) in low- and middle-income countries—either for planning and evaluating interventions, monitoring development over time, or conducting a situation analysis. The Toolkit is intended to help produce reliable, actionable data on child development. Such data are essential at this time. Children in low- and middle-income countries are growing up at a disadvantage. According to estimates in the 2017 Early Childhood Development Series series in The Lancet, more than 250 million children aged under five years worldwide are living in poverty or are stunted and thus are at risk for not fulfilling their potential for physical growth, cognition, or social-emotional development (Black et al. 2017).