The recent advances in the basic hematology and immunology have significantly enhanced the understanding of histiocytic disorders. The Histiocyte Society which was established in 1985 enabled the randomized trials for these diseases, and important knowledge regarding pathogenesis, clinical presentation, diagnosis, therapy and late consequences has been obtained. The treatment of Langerhans cell histiocytosis (LCH) has varied greatly over last decades, and is still controversial. Therapy can be reduced for low risk patients, and it is possible to discriminate early the non-responding patients with risk disease who might require more intensified treatment. Current therapy of LCH recommended by the Histiocyte Society (LCH-III protocol) is activated in 2001. Hemophaocytic histiocytosis (HLH) is fatal if diagnosis is delayed and appropriate therapy is not instituted rapidly. The diagnostic criteria for HLH is revised by the Histiocyte Society for the current treatment protocol (HLH-2004) which consists of dexamethasone, etoposide, and cyclosporin in combination with intathecal methotrexate. Hematopoietic stem cell transplantation is usually necessary for the primary HLH and recurrent secondary HLH.