Langerhans cell histiocytosis (LCH), juvenile xanthogranuloma (JXG), and Rosai-Dorfman-Destombes disease (RDD) can each manifest as a focal lesion, as multiple lesions, or as a widespread systemic disorder with organ involvement. Erdheim-Chester disease (ECD) is a rare systemic disease process in children, with more frequent adult presentations. New distinct emerging entities covered include ALK-positive histiocytosis and post-leukemia/lymphoma histiocytic lesions. These histiocytic lesions are now best classified as myeloid-derived inflammatory neoplastic disorders composed of clonal dendritic- or macrophage-/monocyte-derived cells that infiltrate tissues and are driven by recurrent kinase-activating alterations, most often in the mitogen-activated protein kinase (MAPK), PI3K-AKT, and receptor tyrosine kinase (RTK) signaling pathways, which have all had a long history of being associated with human neoplasia, with ERK overexpression noted in many of these neoplasms (Fig. 28.1; Table 28.1) (1–3). Furthermore, these oncological signaling cascades are critical to the intranuclear regulation of transcription factors that serve as key factors influencing cellular proliferation, survival, and differentiation (4).