Diagnosing chronic diffuse infiltrative lung diseases is often difficult and requires a strict diagnostic approach. The patient's history and in some cases the presence of extrathoracic signs make it possible to distinguish between primary and secondary lung disease (notably pneumopathy). Radiological imaging, functional exploration, bronchoalveolar lavage and histopathology must be performed in a rational order, taking into account their degree of invasiveness and the diagnostic criteria they may provide. When as a result of these investigations the disease has been classified in its aetiological category, its anatomical repercussions must evaluated by non-invasive explorations yielding a true "lesional diagnosis".