ABSTRACTThis review is restricted to neutrophilic granulocytes (granulocytes), monocytes (macrophages), and eosinophils, with only passing reference to cells that are also usually included in the “myeloid” category—megakaryocytes, mast cells, and erythroid cells. Although some dendritic cells are of myeloid origin, they are discussed elsewhere. The validity of the information to be described depends on two assumptions: (a) thatin vitrodata are applicable to eventsin vivoand (b) that mouse data reflect events in man. Both assumptions are likely to be broadly correct.