PROJECT SUMMARY/ABSTRACT Down syndrome (DS) is the leading genetic cause of intellectual disability. Although delayed, cognitive skills develop across childhood and into early adulthood in DS. In adulthood, the study of cognitive functioning in DS shifts from a focus on improvement to a focus on cognitive loss. Because of the triplication of genes located on chromosome 21, individuals with DS are at risk for an earlier onset and increased incidence of Alzheimer’s Disease (AD). In the general population, the diagnosis of AD is preceded by long preclinical and prodromal phases that extend over two decades, and language markers are early indicators of the progression of AD. In individuals with DS, however, gaps in our understanding of development across the lifespan present significant hurdles for research on the progression from normative (albeit delayed) development to the early phases of later cognitive decline. Crucial goals of DS research include elucidating variations in the patterns of development and decline, identifying factors that may be protective and foster resilience against cognitive decline, and developing interventions to support cognitive resilience. Developing measurement tools that that can be used to characterize both growth and decline and evaluating their psychometric properties is vital for understanding disease progression, recognizing potential points of intervention, and evaluating the efficacy of new treatments. In this project, we propose, using a dataset previously collected from 107 participants with DS (6 – 23 years of age), to (1) evaluate the psychometric properties of 9 aging-sensitive spoken language metrics and (2) characterize the developmental trajectories of these metrics relative to participant characteristics to help determine the skills contributing to development and decline. These aims will be addressed by re-transcribing and coding language produced in two expressive language sampling contexts that are the first to be validated for use in DS. Measures derived from the samples will include those indexing lexical and semantic skills, syntactic skills, and verbal hesitations. Test-retest reliability will be assessed over a four-week interval. Standardized measures will be used as indicators of construct validity. A two-year longitudinal follow-up will yield an estimate of relative sensitivity to change of the various measures. The developmental trajectories of these metrics will be explored by describing the performance as a function of participant characteristics (i.e., chronological age, nonverbal and verbal cognition, working memory, adaptive skills, and autism symptomatology). By validating and establishing the psychometrics of these aging-sensitive spoken language metrics in children, adolescents, and young adults with DS, the proposed study will provide tools that can facilitate our understanding of development in DS and identify potential treatment targets to promote resilience against later decline...