Although many children live in healthy, supportive environments, far too many are exposed to or experience biological and/or psychosocial risk factors (e.g., infectious diseases, maternal depression). To evaluate the effectiveness of early childhood programs that are established to support these vulnerable children, funding agencies and nongovernmental organizations are increasing their focus on the development and implementation of interventions aimed at supporting the development of infants and toddlers and need a means for evaluating the effectiveness of the programs. However, there is a lack of psychometrically sound, easy-to-administer, change-sensitive measurement tools to assess the developmental outcomes of children from birth to 3 years. The Ages & Stages Questionnaires: INVENTORY (ASQ:I) is a new measure that was designed to meet this need. The ASQ:I is a continuous measure that was developed by combining items from the Ages & Stages Questionnaires. The ASQ:I is intended to be used for evaluating and monitoring the development of children from 1 to 36 months of age using naturalistic methods that incorporate and capitalize on parent participation. This study presents preliminary evidence for the technical adequacy of the ASQ:I.