Bronchiolitis obliterans with organizing pneumonia (BOOP) is a pathological syndrome common to a variety of pulmonary inflammatory disorders. It is defined by the presence of buds of granulation tissue consisting of fibroblasts and collagen within the lumen of the distal airspaces. BOOP may be secondary to lung injury resulting especially from infection or drug toxicity or may develop in the context of connective tissue diseases or after lung or bone marrow transplantation. BOOP may also be idiopathic (and then preferentially called cryptogenic organizing pneumonia) and then is the hallmark of a distinct clinicoradiological syndrome of subacute pneumonia, with typical alveolar patchy and often migratory pulmonary opacities on imaging. Other imaging presentations consist of diffuse infiltrative opacities or focal pneumonia. Improvement with corticosteroids is usually spectacular, but relapses are common after stopping or while reducing treatment.