Sudden cardiac death occurs in the United States with an incidence of more than 300,000 persons per year. The underlying cause of death is commonly considered to be due to primary or secondary arrhythmias. In young persons in whom no structural heart disease can be identified, the long QT syndromes (LQTS) are commonly considered as likely causes. Multiple genes causing LQTS have been identified thus far, all of which encode cardiac ion channels. These include two potassium channel α subunits (KVLQT1 and HERG), two potassium channel β subunits (minK and MiRP1), and one sodium channel gene (SCN5A). The purpose of this review is to describe the current understanding of the molecular genetics of LQTS and the resultant phenotypes, particularly in young patients.