Implantation models are generally very useful in deciphering mechanisms of the tumor angiogenesis and the role of given angiogenesis stimulator or inhibitor, receptors or modulating intracellular factors. The only drawback is that the adaptive immune system in xenotransplantations is absent and thus the data must be interpreted with care. The other challenge is the implantation site. Usually, it is better to orthotopically implant tumor cells since the correct host tissue is present in this case. Another challenge is whether or not to implant cell suspensions or spheroids. Spheroids seem in many cases better suited because they already adopt a 3D tissue configuration which preserves some of their genetic program that is encountered in a 3D tissue.Co-optive growth may be induced by anti-angiogenic therapy as it is encountered in glioblastoma for instance.