In recent years, several groups have reported the development of antibodies that inhibit or activate Notch signaling. Modulatory antibodies are valuable experimental tools that permit specific targeting of individual Notch receptor homologs (in contrast to pan-Notch-receptor inhibitors like gamma-secretase inhibitors), and show promise as therapeutic agents. Typically, Notch responsive luciferase reporter assays are used to validate and characterize modulatory antibodies. We describe detailed methods for performing dual luciferase-based signaling assays to read out modulation of Notch activity by antibodies designed to inhibit/activate signaling.