This review examines the avian heterophil leucocyte and provides a morphological and cytochemical profile drawn from light and electron microscopy studies of these cells and their characteristic cytoplasmic granules. Other aspects covered include relative and absolute heterophil counts in different avian species and the response of heterophils to stress and to acute inflammation. Heterophils are round cells and, with Romanowsky stains, their primary fusiform granules appear brick-red in colour. A secondary type of round granule, less dense in the electron microscope and smaller than the primary granule, can be seen in most avian species. The primary granules frequently display a ‘central body’ that may be proteinaceous in nature. Unlike mammalian neutrophils, avian heterophils are devoid of myeloperoxidase. However, their cytoplasmic granules contain several lysosomal and non-lysosomal enzymes including acid phosphatase, arylsulphatase, β-glucuronidase, phosphorylase, uridine diphosphate glucose-glucogen glycosyltrans-ferase, neutral and acid α-glucosidases, acid trimetaphosphatase and lysozyme. In the majority of birds heterophils are the second most numerous cell in circulation, the exceptions being several species of the Psittacine and Anseriformes orders, ostrich, ring-necked pheasant, pigeon and rosy flamingo. Heterophils generally outnumber lymphocytes in chicks between hatch and one week of age. Their numbers increase during mildly or moderately stressful conditions and consequently the heterophil/ymphocyte ratio can be used to detect the presence of physiological stress for most stressors. A heteropenia can occur, however, during severe stress. Heterophils respond to a stimulus (chemotactic agent) within about 30 minutes during the early inflammatory phase and they may also have sensitive and selective phagocytosing properties. By seven days heterophils become unrecognizable and, with macrophage recruitment, the characteristic heterophilic granuloma develops.