Myocarditis is an inflammatory disease of the cardiac muscle caused by myocardial infiltration of immunocompetent cells following any kind of cardiac injury. Acute myocarditis in childhood is often a result of a viral infection that produces myocardial necrosis and triggers an immune response to eliminate the infectious agent. Chronic myocardial injury may develop by postinfectious immune or autoimmune processes or be associated with systemic autoimmune diseases, which, in the long run, are responsible for persistent or progressive ventricular dysfunction, arrhythmias, and cardiac complaints. The disease often presents as an acute form of dilated cardiomyopathy, but because of its broad spectrum of presentation the clinical diagnosis is frequently misleading. If the underlying infectious or immune-mediated causes of the disease are carefully defined by clinical and biopsy-based tools, specific immunosuppressive and antiviral treatment options in addition to basic symptomatic therapy may avoid unnecessary interventions and improve prognosis in many patients with acute and chronic disease.