Project Summary/Abstract Hoarding disorder (HD) is a debilitating neuropsychiatric and public health problem that affects up to 4% of the general population and 6% of older adults. Despite formal recognition as an independent diagnosis in the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders, HD remains frequently underdiagnosed and effective treatment is limited. To date, multiple challenges have precluded the development of effective HD detection and intervention efforts, including problems arising from wide-ranging heterogeneity in HD symptom presentation. Efforts to better understand this heterogeneity are lacking, though evidence from other psychiatric disorders indicates extensive utility in identifying dimensional symptom subgroups defined by distinct risk factors and requiring unique prevention and intervention needs. The identification of common symptom patterns may improve our ability to detect clinically meaningful hoarding behaviors and quickly navigate patients to personalized treatment. Thus, the overall goal of this project is to distinguish heterogeneity in HD symptomatology, through which I will establish unique profiles of HD symptoms and assess the utility of symptom subgroups for elucidating variability in disorder etiology and longitudinal course. I will achieve this goal through three specific aims. First, I will identify and characterize HD subgroups on the basis of symptom profile and clutter-related functional impairment and safety concerns. In this aim, I will conduct data-driven analyses using self-report symptom data collected from more than 35,500 individuals who have completed assessments of hoarding and clutter in the Brain Health Registry (BHR), an internet-based research registry of adult participants who semi-annually complete comprehensive self-report assessments of health history and behavior. This rich, epidemiologic data source allows for comprehensive and innovative investigation of HD on a population-level. Second, I will investigate variation in HD etiology following classification of subjects into hoarding symptom subgroups. In this aim, I will employ a social-ecological framework for quantifying subgroup diversity in intrapersonal, interpersonal, and community-level risk factors for HD. Finally, I will assess the differential impact of HD subgroups on the longitudinal course of hoarding symptom severity. In this aim, linear mixed-effects models will be used to assess the predictive ability of HD subgroups for distinguishing symptom trajectory. This work will enrich knowledge of symptom presentation in HD and its relationship to disorder etiology and longitudinal symptom course, thus enhancing efforts to effectively identify and treat hoarding behavior, all the while strengthening my technical and professional skills to support my ultimate career goal as an independent investigator.