This monograph describes the creation of the National Institutes of Health Toolbox Cognition Battery (NIH Toolbox CB) and reports validation data for children ages 3–15 years. Individual chapters described measures of executive function, episodic memory, language, working memory, speed of processing, and attention. Separate chapters were devoted to the factor structure of the test battery and composite measures of cognitive health (Total Composite, Fluid Composite, Crystallized Composite). In all cases, the NIH Toolbox CB measures showed sensitivity to age-related changes across the 3- to 15-year range as well as test/retest reliability. The measures also demonstrated adequate to excellent convergent validity, and there was evidence of greater discriminant validity among older than younger children. Confirmatory factor analyses revealed five- and three-factor models for the older (8–15 year olds) and younger (3–6 year olds) children, respectively. The correlation between the Fluid and Crystallized Composite scores was higher among the younger than the older children. The overall pattern is suggestive of greater differentiation of cognitive abilities with age. The strong psychometric properties of the CB and its apparent sensitivity to patterns of developmental change suggest that it is an important advance in the study of cognitive development and has the potential to substantially accelerate discoveries through use of common methods across disparate laboratories and even disciplines.