Epinephrine QT stress testing is an effective diagnostic tool to unmask concealed Long QT Syndrome (LQTS), particularly type 1 LQTS (LQT1). Unique responses have also been observed in patients with LQT2 and LQT3, making this test invaluable in the diagnostic work-up of LQTS. This article reviews the epinephrine QT stress test, explains the pathological basis of differential responses among patients and healthy individuals, and describes the methodology for conducting the test and the interpretation of the responses. We have also attempted to highlight the differences between the two major LQTS epinephrine QT stress test protocols, the Mayo protocol and the Shimizu protocol.