Bronchiolitis obliterans combined organizing pneumonia (BOOP), now called organizing pneumonia, is a multi-etiologic disease. It can present as a solitary lesion, or as multinodular or diffuse interstitial lung disease. It is speculated if solitary BOOP may evolve into inflammatory pseudotumor of the lung. BOOP can be seen after non-resolving infectious bronchopneumonia as well as acute interstitial pneumonia with diffuse alveolar damage. BOOP can be the early morphologic pattern in toxic inhalation, especially water-soluble substances, but also in drug induced lung disease. BOOP can be the late stage of extrinsic allergic alveolitis, but also a morphologic sequel of collagen vascular disease. Even Wegener's granulomatosis can be preceded by a BOOP pattern. In many cases a careful analysis of BOOP, including changes of the pneumocytes, macrophages, myofibroblasts and endothelial cells, can establish the correct etiologic diagnosis. For example virus-induced pneumocyte proliferation can be seen months after the onset of interstitial pneumonia, and can be found within BOOP. A small percentage of BOOP, however, has to be labeled as idiopathic, which is important too, because of different modalities of therapy. Idiopathic BOOP also is different with respect to prognosis. In the overview different BOOP etiologies will be discussed, and the etiologic background will be analyzed. The pathogenesis will be discussed with respect to the understanding of the causing mechanisms. The role of bronchoalveolar lavage and the optimal tissue sample for establishing the diagnosis will be discussed and demonstrated by examples. A part of the presentation will deal with the differential diagnosis, such as usual interstitial pneumonia, non-specific interstitial pneumonia, constrictive, and respiratory bronchiolitis combined interstitial lung disease.