Heart disease is a leading cause of death in the developed world. Abnormalities of heart muscle (cardiomyopathies) and/or electrical conduction (arrhythmias) are frequent causes of heart failure and sudden death. During the past twelve years, identification of genetic mutations that cause familial cardiomyopathies and arrhythmias has fueled a massive increase in molecular investigation into these diseases. Today, studies of zebrafish mutants with defective heart function are providing insight into the genes required to generate a normal heartbeat.