PROJECT SUMMARY There is a fundamental gap in the availability of behavioral outcome measures that are appropriate, reliable, and valid for use with children with Down syndrome (DS). Lack of such outcome measures represents an important problem to identifying behavioral concerns, measuring treatment effects, and interpreting clinical trials aimed at improving the lives of individuals with DS. Without evidence-based behavioral outcome measures, interventions and treatment trials in this population will remain suboptimal due to poor study measures. Working groups convened by NICHD of leading experts in DS identified that no measure of behavioral concerns was evaluated as appropriate or validated for use with individuals with DS. The overall objective of this application is to develop and validate a novel measure of behavioral concerns, the Behavior Inventory for Down Syndrome (BIDS), that can be used in research, clinical practice, and treatment studies focused on children and adolescents with DS ages 2-17 years, in both English and Spanish. Our rationale for working with this population is that DS is associated with a distinct neurobehavioral phenotype, yet key behaviors of concern common in individuals with DS are omitted from currently available measures of behavioral concerns. Thus, proposed items on the BIDS will incorporate behavioral concerns raised by families and in clinical practice related to children with DS. We propose three specific aims: 1) demonstrate the psychometric properties of the BIDS among children and adolescents with DS in English and Spanish by identifying the BIDS measurement model, establishing reliability (test-retest and inter-rater reliability) and validity (internal and construct validity) 2) evaluating differences in the measure’s psychometric properties as a function of variation in selected demographic and clinical characteristics, and 3) characterize the trajectory of the BIDS over time among children and adolescents with DS to provide comparative data for clinical trials. To achieve these aims, we will conduct online, phone, and paper surveys in English and Spanish, and in-person assessments in English or with Spanish-speaking interpreters, targeting children ages 2-17 years with DS. Our research team is uniquely positioned to conduct this work, combining expertise in DS, measure development/evaluation, Latinx populations, psychological assessment, and comorbid medical conditions. We anticipate that this measurement study will provide critical guidance for future efficacy and effectiveness trials. Our goals are in line with the INCLUDE Project research priority to expand inclusion of individuals with DS in clinical trials by developing appropriate measures of behavior for this population. As our field continues to develop new pharmaceutical and clinical interventions, our project will support this work and have an impact on the 400,000 individuals with DS living in the United States.