The Notch family of proteins plays an integral role in determining cell fates, such as proliferation, differentiation, and apoptosis. We show that Notch-1 and its ligands, Delta-like-1 and Jagged-1, are overexpressed in many glioma cell lines and primary human gliomas. Immunohistochemistry of a primary human glioma tissue array shows the presence in the nucleus of the Notch-1 intracellular domain, indicating Notch-1 activation in situ. Down-regulation of Notch-1, Delta-like-1, or Jagged-1 by RNA interference induces apoptosis and inhibits proliferation in multiple glioma cell lines. In addition, pretreatment of glioma cells with Notch-1 or Delta-like-1 small interfering RNA significantly prolongs survival in a murine orthotopic brain tumor model. These results show, for the first time, the dependence of cancer cells on a single Notch ligand; they also suggest a potential Notch juxtacrine/autocrine loop in gliomas. Notch-1 and its ligands may present novel therapeutic targets in the treatment of glioma.