This monograph presents the pediatric portion of the National Institutes of Health (NIH) Toolbox Cognition Battery (CB) of the NIH Toolbox for the Assessment of Neurological and Behavioral Function. The NIH Toolbox is an initiative of the Neuroscience Blueprint, a collaborative framework through which 16 NIH Institutes, Centers, and Offices jointly support neuroscience-related research, to accelerate discoveries and reduce the burden of nervous system disorders. The CB is one of four modules that measure cognitive, emotional, sensory, and motor health across the lifespan. The CB is unique in its continuity across childhood, adolescence, early adulthood, and old age, and in order to help create a common currency among disparate studies, it is also available at low cost to researchers for use in large-scale longitudinal and epidemiologic studies. This chapter describes the evolution of the CB; methods for selecting cognitive subdomains and instruments; the rationale for test design; and a validation study in children and adolescents, ages 3-15 years. Subsequent chapters feature detailed discussions of each test measure and its psychometric properties (Chapters 2-6), the factor structure of the test battery (Chapter 7), the effects of age and education on composite test scores (Chapter 8), and a final summary and discussion (Chapter 9). As the chapters in this monograph demonstrate, the CB has excellent psychometric properties, and the validation study provided evidence for the increasing differentiation of cognitive abilities with age.