The Notch pathway is an evolutionally conserved cell-cell interaction signalling system involved in several key aspects of cell life, ranging from differentiation and proliferation to apoptosis. As such, it plays an important role in development, homeostasis, angiogenesis and various diseases. Over-activation of the Notch pathway has often been reported in cancer, leading to a variety of effects including increased proliferation, protection from apoptosis and maintenance of cancer initiating cells. Additionally, this signalling pathway has also been involved in tumor angiogenesis. The clearest example of oncogenic Notch signalling is observed in T acute lymphoblastic leukemia (T-ALL), an aggressive neoplasm of immature T-cells, due to genetic alterations leading to ligand-independent increased Notch1 receptor signalling. In solid tumors, however, extrinsic regulation through canonical cell-cell interactions appears to drive activation of the pathway. We recently found that triggering of Notch3 signalling by DLL4 expressed on angiogenic endothelial cells promotes escape of T-ALL cells from tumor dormancy in a xenograft model. This observation discloses un unsuspected role for ligand-dependent regulation of Notch receptors in T-ALL cells, suggesting that blocking extrinsic Notch activation by anti-DLL4 or other ligand-targeted drugs could represent a novel therapeutic approach for this aggressive malignancy.