Congenital long QT-syndrome (LQTS) is a heterogeneous group of cardiac repolarization disorders characterized by QTc-prolongation on an electrocardiogram and a propensity to malignant ventricular arrhythmias. Diagnosing LQTS is challenging due to its heterogeneity. In this thesis, we endeavored to improve the tools for diagnosing LQTS, and to understand why especially children and adolescents are at risk for malignant ventricular arrhythmias. We highlighted that age and sex influence the LQTS-phenotype, most likely as a result of changes in sex-hormones. We further studied these hormonal influences during the menstrual cycle in LQTS-girls and found phenotype-changes associated with pregnanediol levels. We provided age- and sex-specific QTc cut-off values for different methods that are used to measure the QT-interval and to correct it for heart rate. Unfortunately, despite tightening the cut-off values for diagnosing LQTS, a considerable overlap in QTc remains between LQTS-patients and controls. We therefore evaluated the dynamic adaptation of the QTc in adults and children to determine its accuracy in diagnosing LQTS. The LQTS-phenotype can manifest already in the fetus, as we highlighted in this thesis. Because a reliable fetal electrocardiogram is not available, we thoroughly discussed current diagnostic tools. Also, we suggested and investigated potential approaches for diagnosing LQTS prenatally. The results in this thesis emphasize the merit of echocardiographic measures of the relaxation time of the heart in diagnosing LQTS in an early stage. In conclusion, this thesis provides specific recommendations on how to further include phenotypic signs and symptoms for diagnosis and risk-stratification of LQTS, and therefore contributes to optimized tailored medicine and reduced mortality in LQTS-patients in the near future.