PROJECT SUMMARY/ABSTRACT Hoarding Disorder (HD) is characterized by the acquisition of and failure to discard materials regardless of their actual value, resulting in a dysfunctional accumulation of possessions. The disorder affects 2.5% of the U.S. population daily (~6.5 million individuals). HD causes significant functional impairment, reduced quality of life, and represents a substantial public health, safety, and economic burden. The only evidence-based treatment (EBT) for HD is a specialized form of cognitive-behavioral therapy (CBT). The treatment is associated with large effect sizes on average; however, many do not respond due to limited adherence to at-home behavioral assignments. On average, patients with HD complete less than half of their assigned CBT homework. Between-session behavioral avoidance and distress, lack of in-the- moment support and accountability, and limited methods to objectively measure homework completion all contribute to low patient adherence, which in turn predicts treatment non-response. The current Phase I STTR seeks to develop and assess an asynchronous telehealth tool and digital support resource to assist in facilitating delivery of and adherence to CBT for HD. Clutter-GO™ will consist of a web-based clinician portal and patient-side mobile app designed specifically for HD treatment. The platform will allow clinicians to access expert-created EBT homework assignments that can be pushed to patients’ mobile devices. Unlike traditional pen and paper homework lists and logs, Clutter-GO™ will contain a library of step-by-step behaviorally interactive multimedia homework assignments. Thus, patients can do assignments by themselves with increased real-time support and tracking. The platform allows for just- in-time psychoeducation videos, coping skills, emotional processing, and therapeutic accountability via asynchronous connection to therapists, who can monitor objective results immediately. Importantly, a set of image and video capture tools with visual overlays make it easy to track progress over time to promote positive reinforcement. Ultimately, use of Clutter-GO™ is expected to facilitate patient adherence, treatment effectiveness, provider fidelity, and dissemination of EBT for HD. The overall goal of the project is to create a rigorously vetted and highly usable platform for Phase II testing, via four stepwise Aims. Aim 1 is to complete initial development of the product build based on input from internationally known experts in HD treatment. Aim 2 is to iteratively refine the clinician user interface (UI) via consultation and user acceptance testing (UAT) with (N = 20) licensed clinicians who treat HD. Aim 3 is to iteratively refine the patient UI via consultation and UAT with (N = 12) individuals diagnosed with HD. Finally, Aim 4 is to confirm the platform by allowing the expert consultants and licensed clinicians re-test the patient-vetted product for a final round of iterations. The primary outcome crit...