The long QT syndrome (LQTS) is a genetic disorder characterized by prolongation of the QT interval in the electrocardiogram (ECG) and a propensity to torsades de pointes ventricular tachycardia frequently leading to syncope, cardiac arrest, or sudden death usually in young otherwise healthy individuals. The long QT syndrome is caused by mutations of predominantly potassium and sodium ion channel genes or channel-related proteins leading to positive overcharge of myocardial cell with consequent heterogeneous prolongation of repolarization in various layers and regions of myocardium. These conditions facilitate the early after-depolarization and reentry phenomena underlying development of polymorphic ventricular tachycardia observed in patients with LQTS. Obtaining detailed patient history regarding cardiac events in the patient and his/her family members combined with careful interpretation of standard 12-lead ECG (with precise measurement of QT interval in all available ECGs and evaluation of T-wave morphology) usually is sufficient to diagnose the LQTS. Genetic testing plays an important role and is particularly useful in cases with nondiagnostic or borderline ECG findings. The clinical course of patients with LQTS depends on the extent of QTc prolongation and history of cardiac events and is modulated by age, sex, and genotype. beta-Blockers remain the therapy of choice for LQTS, but implantation of a cardioverter defibrillator is increasingly used in high-risk patients.