Endothelial cells have long been viewed as a passive lining of blood vessels endowed essentially with negative properties such as that of being nonreactive to blood components. It is now evident that upon exposure to environmental signals, cytokines in particular, vascular cells undergo profound changes in gene expression and function that allow these cells to participate actively in inflammatory reactions, immunity, and thrombosis. Different mediators (e.g., interleukin-1 [IL-1] and interferon-gamma) activate relatively distinct sets of functions. These functional programs expressed in activated endothelial cells include the production by the same cells of cytokines (e.g., IL-1, IL-6, chemotactic cytokines, and colony-stimulating factors), which regulate hematopoiesis, the differentiation and proliferation of T and B lymphocytes, and the extravasation of leukocytes. The identification of cytokine circuits through which vascular cells participate to thrombotic, inflammatory, and immune reactions provides novel targets for therapeutic intervention.