The terms cryptogenic organizing pneumonia (COP) and idiopathic bronchiolitis obliterans with organizing pneumonia (BOOP) are synonymous. Typical COP should be viewed as a clinicopathological syndrome, consisting of respiratory and systemic symptoms (usually low-grade), patchy consolidation on chest radiography and computed tomography, a restrictive defect on pulmonary function testing, and the presence of buds of granulation tissue within the distal airways and alveolar spaces. In the correct clinical context, the diagnosis can be secured by transbronchial biopsy, with typical bronchoalveolar lavage features providing useful diagnostic support. In typical COP, corticosteroid therapy is highly effective; an early trial of withdrawal of treatment is appropriate because relapse of COP is not associated with a poor long-term outcome. Atypical forms of COP are outlined in this article, including focal, explosive, progressive fibrotic, and nonprogressive fibrotic variants. The occasional evolution of COP into a progressive fibrotic disorder is associated with a poor long-term outcome and poses particular therapeutic difficulties.