Calcium and integrin binding protein 1 (CIB1), a 22kDa EF-hand containing protein, that was originally identified to bind the platelet integrin [alpha]IIb, can also bind and regulate various other proteins. Among the newly identified CIB1-binding partners is the p21-activated kinase 1 (PAK1), which is known to regulate ECs and contribute to angiogenesis in vivo. Since CIB1 is present in highly vascularized tissue and is expressed in various types of ECs, we hypothesized that it may also have an important role in vascular tissue. The work described herein is a collection of studies that for the first time investigates the role of CIB1 in EC signaling, function, and angiogenesis. Pathological angiogenesis contributes to various ocular, malignant, and inflammatory disorders, emphasizing the need to understand this process on a molecular level. We demonstrate here that CIB1 is necessary for various EC functions such as migration, proliferation, tubule formation, and monolayer permeability. CIB1 also regulates PAK1 activation, as well as downstream ERK1/2 phosphorylation and MMP-2 expression. Depletion of CIB1 in ECs attenuates their response to angiogenic growth factors such as VEGF and bFGF. In ex vivo and in vivo assays, CIB1-KO tissue also has an attenuated response to growth factors, demonstrating that CIB1 is necessary for a robust angiogenic response. Moreover, although we confirm that CIB1-KO mice have no defects in developmental vasculogenesis and angiogenesis, we demonstrate that CIB1 is essential for ischemia-induced and tumor-induced pathological angiogenesis. These findings are important since they differentiate between physiological and pathological forms of angiogenesis and identify CIB1 as novel target for pro- and anti-angiogenic therapy.