Accordingly, a step-by-step etiological diagnostic approach is required and is summarized in Figure 3 . Once the diagnosis of ILD is established on clinical, radiological, and functional findings, a careful history should be obtained for potential exposure-related diseases leading to discuss the need for specific serum antibodies against offending antigens. The following step focuses on the search for systemic disease associated ILD, oriented by the presence of clinical and functional extra-pulmonary manifestations. In such situations, additional investigations should include specific serum antibodies and possibly tissue biopsies in organs other than the lung. Finally, elimination of these 2 groups of causes with a lung restricted expression of the disease allows discussing the potential interest of a lung biopsy.