||Almost all vascular diseases are linked to an inflammatory milieu that produces a toxic brew of angiogenic and inflammatory cytokines, which distort tissue architecture and function. These cytokines are produced by a cohort of cells, including endothelial cells, which line blood vessels, and immune cells such as platelets, which are derived from megakaryocytes. Under nonpathological conditions, these cells help to maintain vessel integrity, but when activated by disease states they release cytokines to cause immediate, direct, and disruptive effects on the vascular barrier. In this dissertation I present the results of studies that examine the molecular signaling required to produce one of these immune cells, the platelet, during vessel maintenance as well as examination of the pathological contribution of cytokines on another immune cell, the endothelial cell. I show that under nonpathological conditions, platelets are required for vessel homeostasis, and it is through inhibition of RhoA signaling by proteasomal degradation that platelets are formed. Under pathological conditions, I demonstrate that pro-inflammatory and proangiogenic cytokines signal through a small GTPase, ARF6, in endothelial cells, and that ARF6 is the proximal convergence point regulating downstream signaling leading to endothelial barrier disruption and vessel permeability. This reveals ARF6 as a therapeutic target in vascular diseases such as arthritis and diabetic retinopathy. These studies fundamentally inform a iv mechanism for the genesis of platelets in maintenance of normal barrier function and establish a therapeutic target when vessel homeostasis is disrupted in endothelial cells.