Idiopathic giant cell myocarditis (GCM) and cardiac sarcoidosis (CS) are rare disorders that cause cardiomyopathy, often with ventricular arrhythmias or heart block. Infection, autoimmune processes, and genetics have all been implicated in the pathogenesis of these diseases, but the etiology for both diseases is likely a complex multifactorial process. Both GCM and CS are generally progressive despite treatment with standard heart failure and arrhythmia therapies. Making the diagnosis of GCM or CS on initial clinical presentation is possible in only a small percentage of patients, so myocardial tissue diagnosis is required. The use of multiple noninvasive imaging modalities may aid in diagnosis and assessment of response to treatment. Establishing the diagnosis of GCM or CS early is crucial, as tailored immunosuppressive treatment may significantly alter the clinical course of these patients. The prognosis of patients with GCM is poor, while the prognosis for patients with CS varies according to degree of left ventricular dysfunction.