In the developing CNS, Notch1 and its ligand, Jagged1, regulate oligodendrocyte differentiation and myelin formation, but their role in repair of demyelinating lesions in diseases such as multiple sclerosis remains unresolved. To address this question, we generated a mouse model in which we targeted Notch1 inactivation to oligodendrocyte progenitor cells (OPCs) using Olig1Cre and a floxed Notch1 allele, Notch1(12f). During CNS development, OPC differentiation was potentiated in Olig1Cre:Notch1(12f/12f) mice. Importantly, in adults, remyelination of demyelinating lesions was also accelerated, at the expense of proliferation within the progenitor population. Experiments in vitro confirmed that Notch1 signaling was permissive for OPC expansion but inhibited differentiation and myelin formation. These studies also revealed that astrocytes exposed to TGF-beta1 restricted OPC maturation via Jagged1-Notch1 signaling. These data suggest that Notch1 signaling is one of the mechanisms regulating OPC differentiation during CNS remyelination. Thus, Notch1 may represent a potential therapeutical avenue for lesion repair in demyelinating disease.