This issue of Hematology/Oncology Clinics of North America is devoted to Congenital and Acquired Disorders of Macrophages and Histiocytes.' Guest Editors Nancy Berliner, MD and Barrett Rollins, MD have assembled a group of expert authors to review the following topics: Nosology of Langerhans Cell Histiocytosis (LCH); Cell of Origin of LCH; Genomic Changes in LCH; Clinical Treatment of LCH; Neurodegeneration in LCH; Pathogenesis of Hemophagocytic Lymphohistiocytosis (HLH); Familial HLH; HLH in Adults; Macrophage Activation Syndrome; and Stem Cell Transplant for HLH.