Cell to cell communication plays a role in many physiological processes. A newly discovered route of communication is via Tunneling Nanotubes (TNTs) which are tubular projections that connect cells over long distances (1). TNTs have been shown to facilitate the rescue of stressed cells by allowing the transfer of cellular constituents such as organelles and Ca2+ signals (2). Using high resolution microscopy we have identified multiple types of F-actin containing intercellular connections in human umbilical vein endothelial cells. These can be classified into groups according to their diameter. Fine TNTs are less than 700nm wide. We find that the structures can permit the propagation of mechanically induced Ca2+ signals between cells over long distances (160 μm). In addition there are TNT-like structures that are similar to intercellular bridges observed in epithelial cells and which contain obvious actin bundles and have greater diameters. Staurosporine-induced TNTs were observed in situ in endothelial cells lining the murine aorta. It is concluded that TNTs exist between endothelial cells in intact blood vessels and that they can transfer Ca2+ signals from endothelial cell to endothelial cell over long distances. It is hypothesized that TNTs play important roles in endothelial repair