In this study, the authors examined the psychometric properties and criterion validity of a newly developed battery of tasks that were designed to assess executive function (EF) abilities in early childhood. The battery was included in the 36-month assessment of the Family Life Project (FLP), a prospective longitudinal study of 1,292 children oversampled from low-income and African American families. Ninety-one percent of children were able to complete 1 or more of the tasks. Psychometric analyses were used to test the dimensionality of each task, evaluate the item and task properties, test the dimensionality of the task battery, and evaluate the criterion validity of the battery with multi-informant measures of attention-deficit/hyperactivity disorder (ADHD) symptomatology and child performance on two subtests of the Wechsler Preschool and Primary Scale of Intelligence. Results indicated that the tasks were successful in measuring interindividual differences in child EF ability, that task scores were most informative about ability level for children in the low to moderate range of ability, that children's performance across the entire battery was adequately summarized by a single factor, and that individual differences on the EF battery were related to ADHD symptomatology and intelligence in expected ways. Results are discussed with respect to the importance of developing psychometrically sound, scalable instruments that facilitate the measurement of interindividual differences in intraindividual change of EF across the early childhood period.