Although well characterized in the mouse, the role of Notch signaling in the human T-cell receptor αβ (TCR-αβ) versus TCR-γδ lineage decision is still unclear. Although it is clear in the mouse that TCR-γδ development is less Notch dependent compared with TCR-αβ differentiation, retroviral overexpression studies in human have suggested an opposing role for Notch during human T-cell development. Using the OP9-coculture system, we demonstrate that changes in Notch activation are differentially required during human T-cell development. High Notch activation promotes the generation of T-lineage precursors and γδ T cells but inhibits differentiation toward the αβ lineage. Reducing the amount of Notch activation rescues αβ-lineage differentiation, also at the single-cell level. Gene expression analysis suggests that this is mediated by differential sensitivities of Notch target genes in response to changes in Notch activation. High Notch activity increases DTX1, NRARP, and RUNX3 expression, genes that are down-regulated during αβ-lineage differentiation. Furthermore, increased interleukin-7 levels cannot compensate for the Notch dependent TCR-γδ development. Our results reveal stage-dependent molecular changes in Notch signaling that are critical for normal human T-cell development and reveal fundamental molecular differences between mouse and human.