Objective: To determine the critical assessment characteristics, functional domains, and performance-based measures to include in the National Institutes of Health's (NIH) Toolbox, a uniform assessment battery being developed under the aegis of the NIH's Blueprint for Neuroscience Research for use in clinical trials and cohort studies. Design: Online surveys, in-depth interviews, and systematic evidence-based literature review. Setting: More than 15 collaborating academic and clinical research centers. Participants: 191 experts in clinical and epidemiologic research (Internet survey, n=147; expert interviews, n=44). Interventions: Not applicable. Main Outcome Measures: Survey data were analyzed descriptively using t tests and 1-way analysis of variance to determine assessment priorities and core constructs. Descriptive analysis of expert interview rankings coupled with findings from the evidence-based literature reviews were used to identify appropriate outcomes measures. Results: The full Toolbox should: take less than 2 hours to administer; be precise and accurate across the range of normal function for people aged 3 to 85 years; be easy to administer, score, and interpret; and involve minimal cost. 54 construct areas and appropriate measures for each were identified across 4 core domains of motor function, cognition, sensation, and emotional health. Modifications to standard measures and development of new ones are recommended to meet the Toolbox's inclusion criteria. Implications for rehabilitation research are discussed using the motor and cognitive domains as exemplars. Conclusions: The NIH Toolbox will set the standard for measurement in NIH-funded research; will enable quick, precise measurement of key functional domains; and will facilitate cross-study comparisons.