New blood vessels usually develop in places where they are most needed. A prime example of neovascularization representing a positive feedback response to insufficient perfusion is the development of collateral blood vessels in the ischemic myocardium and leg. The recent discoveries of hypoxia-inducible transcription and angiogenic factors have provided important mechanistic links between the metabolic consequences of ischemia and compensatory angiogenesis. Vascular endothelial growth factor (VEGF) has emerged as the key mediator of ischemia-driven angiogenesis. Environmental stresses, including hypoxia, hypoglycemia, and hypoferremia, upregulate VEGF expression at both the transcriptional and posttranscriptional levels. VEGF acts in turn on adjacent vascular beds expressing cognate receptors and induces sprouting and capillary growth toward the ischemic tissue. In addition to expanding the vasculature at sites where existing vessels have been occluded or obliterated, VEGF also functions to match the vascular density according to developmental and physiologic increases in oxygen consumption. Fine adjustment of the vasculature includes a step of oxygen- regulated vascular pruning mediated by VEGF in its capacity as a survival factor for newly formed vessels. Pathologic settings of ischemia-driven angiogenesis include a major component of stress-induced angiogenesis during tumor neovascularization and abnormal vessel growth associated with retinopathies. The latter represents an excessive angiogenic response to conditions of severe retinal ischemia. Further insights into the mechanism of stress-induced angiogenesis are likely to suggest new ways to augment growth of collateral vessels and to restrain unwarranted neovascularization in tumors and retinopathies.