Changes in executive functioning in the preschool years are recognized as playing a critical role in cognitive and social development, yet comprehensive data and recommendations about measurement of these changes are lacking. The performance of 602 preschool children on several executive function tasks was analyzed and reported as (a) age trends in performance and (b) task difficulty scales at 2, 3, 4, and 5 to 6 years of age. This analysis informs theories of executive function development and offers researchers an evidence-based guide to task selection and design. (author abstract)