Molecular genetic studies have transformed our understanding of the congenital long QT syndromes. Previously, the phenotypes of the Jervell and Lange-Nielsen and Romano-Ward syndromes were characterised by prolongation of the QTc interval greater than 0.44 seconds on the ECG and by syncope or sudden death favorised by stress, the two syndromes being differentiated one from the other by their modes of transmission, recessive or dominant, and by the presence or absence of deafness. Close collaboration between physicians, geneticians and physiologists and the use of inverse genetic techniques led to the localisation of four genes on chromosomes 11, 7, 3 and 4 to the identification of three genes coding for potassium (HERG and KVLQT1) and sodium (SCN5A) channels. These advances have not only improved our understanding of the physiopathology of these syndromes but also our phenotype criteria whilst demonstrating the complexity of clinical diagnosis. They open up new perspectives for the management of patients with these syndromes.