The spectrum of pericardial diseases comprises pericarditis, pericardial neoplasms, cysts, and congenital defects. Due to the insufficient diagnostic value of standard, noninvasive diagnostic techniques, many cases remained etiologically unclear, and were therefore classified as idiopathic. A major improvement in the classification of pericardial disease is its clear distinction between the two most frequent forms of idiopathic pericarditis: viral infection and autoreactive pericarditis. This classification has major therapeutic consequences. In autoreactive forms, systemic and intrapericardial corticosteroid treatment has a favorable effect; its application in viral forms is contraindicated. The new classification of pericardial diseases synthesizes the achievements of modern imaging with molecular biology and immunology. Systematic implementation of new techniques of pericardial fluid analyses, pericardioscopy and pericardial biopsy, and the application of molecular biology and immunology techniques have opened new windows to the pericardial diseases, permitting early specific diagnosis, and creating foundations for etiologic treatment in many cases.