The accumulation of leukocytes in inflamed tissue results from adhesive interactions between leukocytes and endothelial cells within the microcirculation. These adhesive interactions and the excessive filtration of fluid and protein that accompanies an inflammatory response are largely confined to one region of the microvasculature: postcapillary venules. This process is quite complicated and broadly include four distinct components: circulation, adhesion, diapedesis, and migration. First, leukocytes must overcome haemodynamic forces in order to adhere to the endothelial cell surface lining the typical vessel wall. Having done this, they must crawl their way along the endothelial cell surface, migrate through junctions between endothelial cells, and penetrate the basement membrane before gaining entry into, and migrating through the tissue spaces. The nature and magnitude of the leukocyte-endothelial cell adhesive interactions that take place within postcapillary venules are determined by a variety of factors, including expression of adhesion molecules on leukocytes and/or endothelial cells, signaling by cytokines and chemotactic factors, products of leukocyte (superoxide) and endothelial cell (nitric oxide) activation, and the physical forces generated by the movement of blood along the vessel wall.