A clinicopathological disease entity of bronchiolitis obliterans organizing pneumonia (BOOP) is reviewed. Various clinical backgrounds can be found in patients who were histopathologically diagnosed as BOOP. Historically, the idiopathic BOOP has been identified as a clinicopathological disease entity in 1985. BOOP shows clinically interstitial processes, however some confusion is still present in the aspect of differentiation with bronchiolitis obliterans (BO). Histopathologically, BOOP shows organizing reaction to subacute lung injuries, and absence of remodeling separates BOOP from other chronic fibrosing processes such as UIP. Clinically, there is differences in clinical outcomes between idiopathic, and secondary BOOP. We reviewed whether BOOP can be classified as interstitial pneumonia.