This review of the biology peculiar to endothelial cells (ECs) is based mainly on data available within the last decade. The functions described refer to angiogenesis, coagulation, and EC-platelet interaction, inflammation and immune response, synthesis of stromal components, vascular tone regulation, and miscellaneous metabolic activities. Emphasis is placed on the marked variability of ECs from tissue to tissue and from one species to another. This heterogeneity is evident morphologically, functionally, and in the response of ECs to injury.