Mutations involving 6 different ion-channel genes have been identified in subjects with the hereditary Long QT Syndrome. These gene mutations result in structural and functional changes in ion-channel proteins with resultant alterations in potassium and sodium repolarization currents that affect the morphologic features of electrocardiographic repolarization. This review highlights the genotype-phenotype associations related to ventricular repolarization that have been reported in the LQTS literature, with particular focus on ECG T-wave patterns in LQT1, LQT2, and LQT3 genotypes.