The definition and classification of hypertrophic cardiomyopathy (HCM) have varied over the decades, primarily because the phenotypic expression of ventricular hypertrophy can result from a myriad of diseases, especially among children. For the purposes of this article, HCM is a primary cardiac disorder that results from known or suspected genetic defects in sarcomeric proteins of the cardiac myocyte. The disorder is thought to be inherited in an autosomal dominant fashion with variable penetrance and variable expressivity. The hallmark of HCM is myocardial hypertrophy that is inappropriate and often asymmetric and that occurs in the absence of an obvious inciting hypertrophic stimulus. Although any region of the left ventricle can be affected, hypertrophy frequently involves the interventricular septum, which can result in outflow tract obstruction. Patients typically have preserved systolic function with impaired left ventricular compliance that results in diastolic dysfunction, whether or not outflow tract obstruction is present. HCM has a complex set of symptoms and potentially devastating consequences for patients and their families. The clinical presentation and course vary widely; some children are completely asymptomatic, whereas others experience sudden cardiac death. (See Clinical.) In fact, among adolescent children, HCM is the leading cause of sudden cardiac death during exertion.