The critical role of angiogenesis in embryology and tumor biology has been recognized for more than 20 years. However, the fact that neovascularization is essential to processes in mammalian female reproduction has only recently been appreciated widely. In this review we focus on a single angiogenic growth factor, vascular endothelial growth factor. As scientists have discovered in many aspects of cell biology, multiple and redundant signaling pathways have evolved in nature, presumably to protect essential biological functions from inactivating diseases or mutations. Despite this redundancy, some factors are of hierarchical importance. Vascular endothelial growth factor appears to be such a factor in the regulation of angiogenesis.