The cardiovascular system is the first functional organ in the vertebrate embryo, and many organs start to develop adjacent to cells of the cardiovascular system. Endothelial cells (EC) form the inner cell lining of blood vessels and represent the major cell type that interacts with developing organs. On the one hand, EC provide organs with signals. These signals determine the location, differentiation and morphology of an organ. On the other hand, EC receive signals from the organ-specific cell types. Such signals give EC organ-specific features that the organ needs to interact with the circulatory system. This review provides the reader with specific examples of an interdependent development of organs and blood vessels.