A group of disorders involving the excessive proliferation of histiocytes (macrophages) which are not Langerhans cells. Subgroups of this disorder includes hemophagocytic lymphohistiocytosis, sinus histiocytosis, xanthogranuloma and sea-blue histiocyte syndrome. Sometimes only the skin is involved and in other cases there may be systemic involvement which can cause serious problems. See also Histiocytosis, Non-Langerhans-Cell