: The Ages & Stages Questionnaires Third Version (ASQ-3) identifies the risk of developmental delay in children aged 1 to 66 months. The aim of this study was to determine a reliable and valid instrument for the Italian population to enable the screening of children's development. : Data from 2278 Italian children (age range: 1-66 months) were used to evaluate item discrimination power using the corrected item-total correlation. Internal consistency was analyzed by Cronbach's alpha scores and a Confirmative Factor Analysis was conducted to test the factor structure of the test. Data were also collected to examine the ASQ-3 test-retest reliability and concurrent validity, which was investigated using the Griffiths Scales of Child Development, Third Edition, the Peabody Developmental Motor Scale, Second Edition, and the Developmental Profile, Third Edition tools. In order to evaluate discriminant validity, differences between typical development children and several clinical groups have been performed. Finally, two different cut-off scores have been proposed. : The results showed that the questionnaires are composed of high-quality items; the original factor structure has been confirmed and strong Pearson product-moment correlation coefficients between the overall and the total for each domain (ranging from 0.73 to 0.88). The Italian version of the ASQ-3 had adequate internal consistency and a strong agreement between observations with two weeks' intervals. Moreover, the test showed a high discriminant validity due to the possibility of fully discriminating between typical development children and several clinical groups. Finally, two different cut-off scores have been identified using ROC curves in order to have a screening and a diagnostic cut-off value. : This study evaluated the psychometric properties of the Italian adaptation of ASQ-3 questionnaires. We demonstrated the validity of the ASQ-3 and determined new cut-off scores for Italian children. Early identification and accurate assessment are important starting points to better understand and anticipate the needs of children and their link to services.