The endothelium is a single layer of cells lining all blood vessels. Although the endothelium is not an organ it does, nonetheless have autocrine, paracrine and endocrine-like functions that affect the cardiovascular system. Until the description in 1980 by Nobel Laureate Robert Furchgott of endothelium-derived relaxing factor (EDRF), later identified as nitric oxide (NO), the endothelium was considered to be a semi-permeable barrier between the blood and the smooth muscle cell layers of the blood vessel. Heterogeneity exists in the functions of the endothelium with differences evident between species and between the large conduit compliance vessels and the resistance vessels of the microcirculation. Endothelial dysfunction, defined as a reduction in the ability of the endothelium to transmit a vasodilatation influence on blood flow, has prognostic significance and serves as an early indicator of the development of vascular disease as well as a therapeutic target. This review compares the role of the endothelium in the regulation of vascular tone in conduit versus resistance vessels and how alterations in endothelial function may lead to vascular disease.