Chronic interstitial lung diseases (ILDs) are characterized by: 1) diffuse interstitial inflammation and/or fibrosis, and 2) duration of illness above 3 months. Chronic ILDs of known and unknown aetiology have to be differentiated. The diagnosis of chronic ILDs, which is suggested by clinical and/or pulmonary function test data, is definitely based on the identification of interstitial changes on chest-x ray or thoracic high resolution CT scans. Orientated laboratory evaluation and flexible bronchoscopy with bronchoalveolar lavage and biopsies are also performed. These investigations often allow a secure diagnosis to be made. Sometimes however, a surgical lung biopsy is necessary for the diagnosis, notably when chronic idiopathic interstitial pneumonias are considered.