The number of recognized genetic mutations capable of causing congenital lengthening of the QT interval with the associated ventricular arrhythmias increases almost yearly and, at this writing, 15 mutations have been recognized. However, approximately 95% of all recognized cases are due to 3 genetic abnormalities and of these, the vast majority are the two which involve subunits of the genes responsible for the channels that regulates the potassium outward currents responsible for repolarization (phase 3 of the action potential). The third involves a subunit of the gene that regulates the rapid sodium inward current. These 3 genotypes, referred to as types 1,2, and 3 are important to recognize clinically because their presentations and therapies differ. Types 1 and 2 are due to abnormalities in the K channels and are characterized electrocardiographically by notching and lengthening of the T wave, whereas Type 3 is due to an abnormality in the Na channel that is reflected electrocardiographically by lengthening of the ST segment.