Once thought to be a monolayer of passive cells lining the vasculature, endothelial cells are now known to be important regulators of normal vascular physiology. Unfortunately, these critically important cells are destroyed or removed by interventional and surgical procedures performed to recanalize or bypass vascular obstructions. The loss of these cells contributes to thrombosis and restenosis, the major complications observed after angioplasty, stent deployment, and prosthetic graft implantation. One approach to preventing these complications is the placement of endothelial cells on stents or prosthetic grafts prior to their placement in vivo in the hope that these cells will, after growth and maturation, release the factors necessary to inhibit thrombosis and intimal thickening. The purpose of this review paper is to provide an overview of the physiologic functions of normal and dysfunctional endothelial cells, and to discuss experiments in which endothelial cells have been placed on metallic stents and prosthetic grafts.