PROJECT SUMMARY/ABSTRACT Histiocytic disorders, or histiocytoses, are a family of rare hematologic diseases encompassing the histiocytic neoplasms (including Langerhans cell histiocytosis, Erdheim-Chester disease, and Rosai-Dorfman disease) and the hemophagocytic lymphohistiocytosis (HLH) disorders. Histiocytosis affects both adults and children, and clinical disease is mediated by accumulation of macrophage, dendritic cell, or monocyte-derived cells in affected tissues. These are orphan diseases with an aggregated prevalence of fewer than 10,000 patients in the US. Despite important recent advances in histiocytic disorders, key knowledge gaps remain owing to their rarity and to the absence of collaboratively executed investigation across adults and children and across the disease subtypes. Specifically, there is a lack of unified approach to observational and natural history studies, as well as a coordinated approach to the prioritization and implementation of therapeutic trials. Additionally, there is an urgent need to engage the community of stakeholders in histiocytosis investigation in order to elevate its patient-centeredness and ensure that research is responsive to the community’s needs. Last, there is a pressing need for the histiocytosis scientific community to appraise its own composition and to take a deliberate, inclusive approach towards incorporating diverse perspectives into the vision and execution of future research. The proposed Histiocytosis Disorders Workshop (HDW) represents an unprecedented gathering of senior and junior scientists—across adult and pediatric diseases, HLH and histiocytic neoplasms—eager to align priorities and approaches to research in these diseases. The two-day event will include approximately 50 in-person diverse attendees including histiocytosis scholars, patients and stakeholders, experts in (a) patient engagement and patient-reported outcomes, (b) diversity and inclusion, (c) junior career development, and representatives from NIH, FDA, and PCORI. The workshop will include lectures, moderated panels, and breakout sessions on the topics of observational studies, therapeutic trials, biomarkers and response assessment, patient reported outcomes, community engagement, faculty development and inclusiveness, and diversity and inclusion. The event will be advertised by social media and email to scientific societies and patient and community groups with emphasis upon inclusion of junior investigators and attendees from diverse communities. The HDW is timely in (1) its integration of the most current and advanced science related to histiocytosis and (2) its leveraging the North American Consortium for Histiocytosis (NACHO), a mature and operational consortium for pediatric histiocytosis to host and implement the meeting. The workshop will be useful to the scientific community by defining (1) key data elements for observational studies, (2) strategies for prioritizing and implementing therapeutic trials and (3)...