Current ideas about the histiocyte-macrophage system are briefly reviewed and the functions attributed to the system are defined. An anatomico-clinical classification of histiocytic diseases into four categories is suggested: - secondary of associated histiocytosis illustrating, in particular, macrophage function in immune response; - histiocytosis due to dystrophia or overload illustrating mainly phagocyte function and ability to store fats; - Langerhans histiocytosis characterized by the presence of numerous Langerhans cells. - Neoplastic histiocytosis, which is the only genuine malignant tumoral proliferation of the histiocyte-macrophage system.