The diagnosis of myocarditis is difficult because there is no pathognomonic clinical presentation and the disease may mimic other non-inflammatory diseases. Thus, current classifications on cardiomyopathies (e.g., the World Health Organization and the International Society and Federation of Cardiology [WHO/ISFC], the European Society of Cardiology [ESC], and the 2013 Expert Myocarditis ESC Task Force) define myocarditis as an inflammatory disease of the myocardium, which is diagnosed on endomyocardial biopsy (EMB) based upon histological, immunological, immunohistochemical and molecular tools. This will identify etiology, and differentiate between infectious, mainly viral, and non-infectious, immune-mediated forms. The term "inflammatory cardiomyopathy" may be applied in biopsy-proven myocarditis with associated left, right or biventricular dysfunction. Myocarditis may resolve spontaneously, relapse or become chronic progressing to dilated cardiomyopathy, death or heart transplantation. The 2013 Myocarditis ESC Task Force consensus document recommends consideration of EMB and selective coronary angiography in all patients with clinically suspected myocarditis according to the Task Force criteria. It is recommended that EMB analysis includes not only histology (Dallas criteria), but also immunohistology and detection of the genome of infectious agents by molecular tools. EMB should be performed by expert teams. The rationale for this diagnostic effort is the availability of a wide range of immunosuppressive or immunomodulatory agents that, as shown in systemic extracardiac autoimmune disease and in many clinical studies, can be used in infection-negative myocarditis patients to stop or at least stabilize chronic cardiac tissue damage mediated by the immune system, and thus prevent fibrosis and progression to irreversible end-stage dilated cardiomyopathy.