Pericarditis, as an inflammation of the pericardium, is part of the pericardial syndrome of pericardial diseases. It also represents a syndrome itself, which can differ in its etiology and clinical manifestation over time. Thus, acute, persistent (“incessant”), recurrent, and chronic pericarditis are distinguished. The clinical picture ranges from dry pericarditis to cardiac tamponade or constrictive pericarditis. Etiologically, infectious forms are distinguished from non-infectious ones. The therapy initially aims at eliminating the symptoms and taking into account the etiology. Therefore, pericardial puncture is indicated for larger and tamponading effusions. In addition to the life-saving relief of the pericardial effusion, the effusion and the peri- and epicardial biopsy can be processed cytologically, histologically, and immunologically to create the prerequisites for later causal therapy. Pericardial diseases refer to inflammatory and non-inflammatory changes in the double layer of the pericardium, the visceral pericardium (epicardium) lying on the myocardium, and the outer parietal pericardium.