Organizing pneumonia (OP) is a histologic term characterized by patchy filling of alveoli and bronchioles by loose plugs of connective tissue. OP may be an incidental finding in lung biopsy specimens or may be found nearby areas of lung involved by other diseases. On other occasions, OP may be the primary cause for pulmonary dysfunction and/or pulmonary symptoms. OP can be either idiopathic (cryptogenic organizing pneumonia, COP) or secondary to underlying disease (secondary organizing pneumonia, SOP). COP typically presents with a prodrome of symptoms of a respiratory illness followed by the insidious onset of dyspnea weeks to months later. The radiological findings typically reveal peripheral consolidation, although ground glass infiltrates or solitary nodules may be seen. The definitive diagnosis of OP requires histology. Open lung biopsy or video assisted thoracoscopy is usually required to obtain specimens large enough for the diagnosis to be made. In some cases, transbronchial biopsy specimens may be adequate for the diagnosis. The treatment of choice for OP includes corticosteroids plus treatment of the underlying disease in cases of SOP. Relapses occur frequently, usually when treatment is withdrawn or tapered. The prognosis is good in most of the cases of COP, whereas in SOP it is dependent on the underlying cause.