Examined the Ages and Stages Questionnaires (ASQ), a series of 11 developmental questionnaires designed to be completed by parents and caregivers of young children from 4 to 48 months of age. The ASQ were recently revised and additional psychometric data were gathered. Analyses on over 7,000 questionnaires indicated high test-retest reliability, interobserver reliability, and internal consistency. Concurrent validity using standardized measures yielded on overall agreement of 85%, with a range of 76-91%. Specificity was high across questionnaire intervals while sensitivity was lower and varied across intervals. Use of parent-completed screening tools such as the ASQ is attractive in terms of cost-effectiveness, parental involvement, and flexibility in administration procedures.