Summary/Abstract Children with Down syndrome (DS) experience significant language deficits beginning early in life and persist- ing throughout adulthood. Language deficits negatively impact academic outcomes and quality of life. Most children with DS participate in early language and communication interventions, which frequently include a caregiver-mediated component. In early interventions, caregivers are often instructed to adapt their language input to match or slightly exceed their child’s spoken language abilities. However, currently there is no empiri- cal evidence to indicate what level of linguistic simplification optimizes language processing and word learning in young children with DS, leading to a substantial gap in clinical practice. This information is particularly im- portant given that children with DS present with uneven language profiles. Specifically, children with DS show relative strengths in receptive over expressive language, and in lexical over morphosyntactic skills. Thus, modi- fying language input based on the child’s expressive language level may result in input that is too simplified for the child’s receptive language abilities, potentially inhibiting language development in a population that already struggles to learn language. The proposed project adds a sample of 30 young children with DS, ages 2—5 years, to an ongoing R01 that examines how linguistic simplification affects language processing and word learning in young autistic children. The methodology is consistent across participant groups and includes using looking-while-listening tasks to examine the effects of linguistic simplification on language processing and word learning in real-time. Looking-while-listening only requires passive participation (i.e., looking), eliminating most behavioral demands and allowing for testing of participants with a range of cognitive and linguistic abilities. Participants will also complete standardized language assessments. The central hypothesis of the INCLUDE supplement is that simplification will negatively affect processing and learning. This central hypothesis will be addressed by 3 Specific Aims, which mirror the aims of the parent R01 but in a sample of young children with DS: 1) Determine how single-word and telegraphic simplification affects language processing in young children with DS. 2) Determine how single-word and telegraphic simplification affects word learning in young children with DS. 3) Evaluate child characteristics that may moderate the effects of linguistic simplification on language processing and word learning in young children with DS. Regardless of the outcome, these results will have a strong impact on theory and clinical practice. Project results will identify how linguistic simplification affects chil- dren’s ability to understand and learn from spoken language. The proposed supplement aligns with INCLUDE’s research plan, particularly with regard to Component 3, as it includes a clinical trial th...