The sudden stoppage of a young person’s heart (“heart arrest”) is very upsetting for the victim’s family, friends, and community. In the majority of cases, the heart stops pumping when it goes into a dangerous rhythm and in almost all cases, the young people die. Recent reviews have highlighted our lack of information on how many young people die every year, including the cause of why their heart suddenly stopped and if the cause is due to something in their genes. We want to understand why certain young people’s hearts suddenly stop functioning, in order to see what can be done to prevent these events from happening in the first place. We think that by figuring out how many young people die suddenly each year in the Greater Toronto Area as well as why their hearts suddenly stopped might help prevent events in family members at risk.