The endothelium is not a passive blood-compatible lining for the containment of blood cells and plasma, but rather it is a metabolically active tissue that subserves a wide range of functions relating to vascular homeostasis. This article reviews the current understanding of endothelial cell biology in terms of the molecules and biochemical pathways involved. These regulate coagulant and thrombotic properties of the vessel wall, vascular tone, and hence blood flow and pressure; changes in solute permeability and leukocyte traffic during the generation of inflammatory and immune responses; and finally the processes of vessel growth and angiogenesis. The review concludes with a consideration of how these functional properties can be disturbed, and their possible consequences, in response to irradiation, intravascular contrast media, or angioplasty.