The vascular endothelium is an indispensable organ in the regulation of tonicity and vascular homeostasis. Endothelial dysfunction is a marker of atherosclerosis and contributes to the atherogenic process and the development of atherothrombotic complications. Endothelial dysfunction is the common meeting point among factors for vascular risk and launches complex cellular and biochemical interactions that are characterized by oxidative stress and an intense inflammatory response. The strategic importance of the concept of endothelial dysfunction arises from the following facts: its diagnosis is simple, it allows the identification of asymptomatic arteriosclerotic subjects with a high risk of developing atherothrombotic complications, it is reversible, and it responds to various therapeutic strategies. 2005 S. Karger AG, Basel