The chick embryo chorioallantoic membrane (CAM) is an extraembryonic membrane mediating gas and nutrient exchanges until hatching. Since it has a dense capillary network, it has been commonly used in vivo to study both angiogenesis and antiangiogenesis in response to normal tissues and cells, to tumor bioptic specimens and cells, or to soluble factors. During the last 8 years, this assay has been used in over 550 published works. This chapter summarizes current knowledge about the embryological origin of the CAM, morphology of its blood and lymphatic vessels, the use of CAM in the study of tumor angiogenesis and metastasis, angiogenic and antiangiogenic substances. The angiogenic response of CAM to multiple myeloma and neuroblastoma cells and bioptic specimens and their responses to antiangiogenic molecules and the role played by fibroblast growth factor-2 in CAM vascularization have been analyzed in detail. Finally, advantages and limitations of CAM as an experimental model to study angiogenesis and antiangiogenesis are discussed.