ABSTRACT Down syndrome (DS) is the leading genetic cause of intellectual disability. Moreover, virtually all aspects of cognitive and behavioral development show very slow rates of development relative to age expectations in people with DS. Life expectancy for individuals with DS has increased in recent decades; however, DS is also characterized by early aging-related declines and a high risk for Alzheimer’s Disease (AD). Thus, there is a need in the DS field to develop treatments that accelerate development during childhood and beyond as well as treatments that slow or prevent the inevitable declines associated with AD. Evaluating the efficacy of such treatments, however, requires psychometrically adequate measures of change. The Expressive Language Sampling-Narrative (ELS-N) procedure has been shown to be a psychometrically sound measure for tracking expressive language growth with age or within treatment in children, adolescents, and young adults with DS. There is also preliminary evidence that ELS-N is sensitive to some early signs of decline. However, ELS-N has not been validated in older adults with DS; has not been evaluated for non-English speakers; and has been tested only in a clinic-based, clinician-administered format. To make ELS-N accessible to a wider array of diverse participants, we have developed, and pilot tested, English and Spanish versions of ELS-N that can be administered by caregivers to individuals with autism at home after telehealth-delivered training to caregivers. In the proposed project, we further expand ELS-N by recruiting 60 verbal individuals with DS, ages 10 to 40 years, along with their caregivers, with equal numbers of English and Spanish speakers. Two ELS-N delivery methods are proposed: (a) at-home, caregiver-implemented (after telehealth training) and (b) the “classic” version of ELS-N (i.e., in-clinic, professional-administered). Each participant with DS will receive the two ELS-N delivery methods. The specific aims of the project are: (1) examine the consistency of performance individuals with DS across the two ELS-N delivery methods; (2) examine practice effects, test-retest reliability, and construct validity for the variables derived from each ELS-N delivery method; and (3) examine the consistency of performance and psychometric properties of the ELS-derived variables separately for ages 10-20, 21-30, and 31-40 years, although this last aim is exploratory due to limited sample size. The aims will be addressed for the sample as a whole and for each language group separately, thereby providing data relevant to future studies involving either multi-language or single-language samples of participants. This project will expand the array of outcome measures available for assessing treatment efficacy across the lifespan of individuals with DS, increase the potential for broader representation in research and improve equitable access to care, and identify potential linguistic indicators of both growth and decl...