Long QT syndrome is one of the leading causes of sudden death in young people, with a prevalence exceeding 1 in 2000.[1][2] It is characterized by prolongation of the QT interval on the electrocardiogram at rest, abnormal T wave shapes and susceptibility to life-threatening arrhythmias, which are mainly triggered by adrenergic activation.[1][2] Pathogenic variants causing disruption of IKs and IKr currents prolong the QT interval.[1] Examples include a QTc of 673 ms, serrated T waves typical of type 2 with a QTc of 483 ms, or a QTc of 640 ms in a relapse after an exercise test.[1] Therapy includes beta-blockers, in at-risk patients a combination with mexiletine or ICD implantation.[1][3] The risk of arrhythmias and treatment should be reassessed annually to optimize therapy.[1][3] The article summarizes the genetics, diagnosis and treatment of this syndrome.[2][4]