CBFbeta is the non-DNA binding subunit of the core binding factors (CBFs). Mice with reduced CBFbeta levels display profound, early defects in T-cell but not B-cell development. Here we show that CBFbeta is also required at very early stages of natural killer (NK)-cell development. We also demonstrate that T-cell development aborts during specification, as the expression of Gata3 and Tcf7, which encode key regulators of T lineage specification, is substantially reduced, as are functional thymic progenitors. Constitutively active Notch or IL-7 signaling cannot restore T-cell expansion or differentiation of CBFbeta insufficient cells, nor can overexpression of Runx1 or CBFbeta overcome a lack of Notch signaling. Therefore, the ability of the prethymic cell to respond appropriately to Notch is dependent on CBFbeta, and both signals converge to activate the T-cell developmental program.