A prominent J wave is encountered in a number of life-threatening cardiac arrhythmia syndromes, including the Brugada (BrS) and early repolarization (ERS) syndromes. BrS and ERS differ with respect to the magnitude and lead location of abnormal J waves and are thought to represent a continuous spectrum of phenotypic expression termed J wave syndromes. Despite two decades of intensive research, risk stratification and the approach to therapy of these two inherited cardiac arrhythmia syndromes are still undergoing rapid evolution. Our objective in this review is to provide an integrated synopsis of the clinical characteristics, risk stratifiers, as well as the molecular, ionic, cellular and genetic mechanisms underlying these two fascinating syndromes that have captured the interest and attention of the cardiology community in recent years.