PROJECT SUMMARY Adults with Down syndrome (DS) are twice as likely to be obese (BMI > 30kg/m2) than their peers without DS. The health consequences of excessive adiposity include greater risk for cardiovascular diseases, cancers, type II diabetes, etc. These alarming health implications highlight the importance of identifying accurate methods for body composition and physical activity (PA) analysis since an early detection and intervention could help alleviate the burden of diseases that occur later in life. Unfortunately, many methods used for assessing body composition and PA may be perceived as threatening and burdensome by individuals with DS. Particularly, the need to visit a research laboratory or clinical testing center may induce discomfort, as our team has noted in previous research conducted in adults with DS. The MPI’s can attest firsthand that many subjects with DS find body composition testing as threatening when using devices such as dual energy X-ray absorptiometry (DXA). Several subjects with DS that visited our laboratory had difficulty remaining motionless during the 5-to-10- minute DXA scanning procedure. We overcame those issues by having a parent or guardian available to offer verbal encouragement during the scan. Nonetheless, the emergence of wearable technology may offer the opportunity to measure body composition and PA in a comfortable environment. As a result, the utilization of wearable technology for body composition and PA may overcome problems commonly observed by researchers in validation studies. This multi-site cross-sectional study will include 50 adults with DS (18-60 years of age). Body composition will be measured via the InBody Band 2 (biompedance-based smartwatch) and compared against a criterion DXA scan. Further, the validity of wearable PA trackers will be examined via the following devices: wrist – Samsung Galaxy Watch 4, InBody BAND 2; waist – ActiGraph GT9X, thigh – activPAL Micro 4; and ankle – StepWatch 4. The wearable PA trackers will be validated against a criterion measure that consists of directly observed, hand tallied step count (with video recording for step count verification). This project relates to the mission of NIH since knowledge gained will be useful in promoting the health of a special population that has an increased risk of obesity by identifying whether a wearable technology can be used in weight management and PA interventions. In addition, this study is responsive to NOT-OD-21-092, which will study the validation of at-home diagnostic tools including actigraphy and wearable technologies in individuals with DS.