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After more than 30 years of research into the hypothesis that long QT syndrome (LQTS) might be a cause of arrhythmic sudden infant death, we are now at the point where we can state with certainty that some sudden unexplained deaths in infancy, about 10%, are indeed due to long QT syndrome. The evidence for this lies in large population ECG screening programmes, post-mortem molecular genetic testing of sudden infant death victims, and some informative case reports. The cardiac sodium channel gene SCN5A (LQTS type 3) is the most common culprit, but LQTS types 1,2, 6, 9 and 12 have also been found. There is also new evidence that other arrhythmic syndromes sometimes cause SUDI, in particular short QT syndrome, and catecholaminergic polymorphic ventricular tachycardia (CPVT). These conditions are also due to disordered cardiac ion channel function like LQTS, and are usually inherited in an autosomal dominant fashion. There remain, however, many unanswered questions, most particularly whether all populations are affected equally, and what should clinicians do with this knowledge? Should newborn ECG screening become mandatory? How should we best investigate SUDI at post mortem in order to diagnose LQTS? This review summarises the evidence to date and addresses these questions.