The vascular endothelium has long been considered to be a homogeneous cell type. It is only recently that its remarkable heterogeneity has been shown. The selective and organ-specific adhesion of cells of the immune system, of metastasizing tumor cells and of microorganisms underscores the central position of the endothelium between the circulation and each organ. The use of antibodies and lectins led to the recognition of species-, organ-, and age-specific endothelial cell surface receptors. Identification of the endothelial adhesion molecules and -mechanisms, which are part of the "endothelial organ- and tissue address" would lead to the possibility of blocking the adhesion of cells, such as e.g. tumor cells.