There have been many attempts in the past to classify phagocytic mononuclear cells and to define the cell system they are considered to form—among these being the “macrophage system” of Metchnikoff, the “reticulo-endothelial system” of Aschoff, and the “reticulo-histiocyte system” proposed by Volterra and reintroduced by Thomas. None of these is entirely adequate in the light of present knowledge. In 1969, therefore, a group of workers proposed a new classification of all highly phagocytic mononuclear cells and their precursors in what they termed the “mononuclear phagocyte system”. This system includes the promonocytes and their precursors in the bone marrow, the monocytes in the peripheral blood, and the macrophages in the tissues. Subsequent consultation with numerous other specialists throughout the world led to a certain number of changes in this classification, which is now proposed in revised form.