Diffuse interstitial lung diseases (DILD) cover more than 200 conditions classified in 4 groups: secondary DILD; sarcoidosis; particular DILDs and idiopathic interstitial pneumonias. Overall, these diseases have a prevalence of 60-80/100,000 and an incidence around 30/100,000. Sarcoidosis and idiopathic pulmonary fibrosis are the 2 most frequent diseases, accounting together for more than 50% of all cases, followed by DILD related to connective tissue disease and to immunologic lung diseases. The incidence of different causes of DILD depends on epidemiologic factors: age, sex, race, smoking habits, and history. Mortality is very elevated in idiopathic pulmonary fibrosis, linked in more than 80% of cases to the fibrosis itself, with a median survival between 24 and 36 months. The epidemiologic studies on DILD have substantial limitations, and new studies must be conducted, in particular in France where we lack epidemiologic data beyond one old study of sarcoidosis and one more recent study focused on farmer's lung in the Doubs.