Notch proteins are used repeatedly to direct developmental cell fate decisions in multiple organs. During hematopoiesis and immune development, Notch is critical for T/B lineage specification and for generation of splenic marginal zone B cells. In early embryonic development, Notch is crucial for generating hematopoietic stem cells. Emerging data suggest that Notch may also modulate the differentiation and activity of peripheral T cells. Understanding the specific regulation of the Notch pathway in different contexts and its interaction with other signaling pathways remains an important challenge to comprehend the full spectrum of Notch effects. In this review, we critically assess recent findings regarding the function of Notch in the hematolymphoid system.