Cardiomyopathy is a disease of the heart muscle. It is rare, but it can be serious. Cardiomyopathy in children can result in death, disability, heart transplantation or serious heart rhythm disorders. Natural substances in the blood called cardiac biomarkers can be measured in the laboratory and could be a less invasive way (compared to echocardiograms or MRIs) to detect heart dysfunction in children with cardiomyopathy. Little is known about how useful and valid cardiac biomarkers are in the diagnosis and determination of the symptoms in children with cardiomyopathy. The long-term goal of this project is to study how helpful measuring cardiac biomarkers in children with cardiomyopathy is to their doctors in managing the care of these patients as well as improving their overall health. Measures of these cardiac biomarkers could help doctors in determining how best to care for a child with cardiomyopathy, including when to consider heart transplantation as a treatment option.