Signals transduced by Notch receptors influence differentiation and proliferation in a wide variety of cell types. Activation of a Notch signal by one of several ligands triggers a series of proteolytic cleavages that release the intracellular region of Notch from the membrane, allowing it ultimately to translocate to the nucleus and activate the transcription of downstream target genes. Recent studies have elucidated the roles of several key proteins that participate in and modulate these central events in Notch signal transduction. These advances offer a variety of potential avenues to manipulate Notch signaling for therapeutic purposes in the treatment of cancer and in stem cell maintenance.