J wave syndromes represent a spectrum of electrocardiographic manifestations of ventricular repolarization abnormalities characterized by prominent J waves and the potential to predispose affected individuals to polymorphic ventricular tachycardia (VT) or ventricular fibrillation (VF). J wave syndromes include two major forms: early repolarization syndrome and Brugada syndrome. These syndromes share a common ionic and cellular mechanism with prominent I to -mediated action potential (AP) spike and dome in ventricular epicardium and are linked by J wave on the ECG. When I to is large, an increase in net repolarization current by either a decrease in an inward current (I Na or I Ca ) or an increase in an outward potassium current (I to per se or I K-ATP ) predisposes loss of epicardial AP dome. Loss of epicardial AP dome is often heterogeneous, resulting in two important changes at cellular levels that facilitates the development of polymorphic VT/VF: (1) the epicardial AP dome in some sites may propagate to sites where the cells has completely lost their dome, leading to phase 2 reentry that manifests short-coupled extrasystoles on the ECG; (2) an increase in transmural dispersion of repolarization that can serve as reentry substrate and manifest as downslope ST segment elevation. This chapter attempts to translate the abnormalities at cellular level associated with J wave syndromes to their clinical ECG manifestations. © Springer International Publishing Switzerland 2016.