Pulmonary fibroses are terminal stages of exposure to a whole series of very diverse noxae. Both occupational and nonoccupational causes must be distinguished. Finally, there is the large collective category "idiopathic" fibroses. In terms of their systematics, fibroses can be classified in five largegroups. The classification criteria are etiological when the causes are known, and morphological-descriptive in cases of idiopathic fibrosis. Occupational and "idiopathic" fibroses often cannot be distinguished in biopsy material because of their great histomorphological similarity. In such circumstances, the result of further analyses is crucial. For this reason, the possibility that an occupational pulmonary fibrosis is present must also be considered even in "idiopathic" pulmonary fibroses in order to arrange for further analyses such as BAL, ashing and energy-dispersive X-ray analysis to be performed.