Histiocytic disorders are a group of heterogeneous diseases. A logical classification can be based on the type of proliferating cell, either monocyte-macrophage or Langerhans/dendritic cell, and depends whether the proliferating cells are "reactive" or malignant. The classification now mainly depends on the histological examination. Regarding Langerhans cell histiocytosis (Hand-Schüller-Christian disease, Letterer-Siwe disease and eosinophilic granuloma), the diagnosis suspected on various clinical signs, is confirmed with histological examination showing infiltration with CD1 positive histiocytes disclosing intracytoplasmic Birbeck granules at electron microscopic examination. The prognosis depends on the patient's age at onset and the extension of the disease. Treatment is based on chemotherapy and corticotherapy.