Histiocytic disorders consist of many rare related and unrelated wide-ranging proliferations of cells supposedly derived from or sharing common immunophenotypes with macrophages (such as hemophagocytic lymphohistiocytosis) and dendritic cells (such as juvenile xanthogranulomas). These lesions can involve virtually any organ, resulting in an assortment of clinical presentations and prognostic outcomes that vary from localized incidental self-limiting lesions to multisystemic potentially fatal conditions that require chemotherapy or other aggressive treatments. Recent advances in our understanding of histiocytic lesions have shed new light on the pathophysiology of histiocytic disorders, revealing that many distinct entities have overlapping mutations of BRAF or other genes in the MAPK pathway [1]. In 2016, the Histiocyte Society proposed a revised classification system in which histiocytic disorders are sorted into five groups (summarized in Table 22.1) based on clinical, radiologic, histopathologic, immunophenotypic, and genetic/molecular characteristics [2].