Organising Pneumonia (formerly called Bronchiolitis Obliterans with Organising Pneumonia) is a particular form of inflammatory and fibroproliferative lung disease. Its idiopathic form called Cryptogenic Organising Pneumonia, was recently defined by an ATS/ERS consensus conference. The disease onset is subacute with cough, dyspnea, fever, asthenia, weight loss, crackles, and elevation of biological inflammatory markers. Bronchoalveolar lavage reveals a mixed alveolitis with elevated lymphocyte, neutrophil, and eosinophil counts. Chest imaging usually shows multifocal alveolar opacities predominating in the subpleural regions, often with a migratory pattern. Lung biopsy reveals budding connective tissue filling the distal airspaces. Diagnosis is established by combining clinical, radiological and histological criteria. Similarities with other disease processes can lead to delayed or erroneous diagnosis. Most patients respond well to corticosteroid therapy. Relapses are frequent but can generally be controlled with moderate doses of prednisone and do not worsen the prognosis. The therapeutic strategy aims at reducing the steroid doses while maintaining an optimal disease control.