Interstellar medium and infrared emission of the galactic disc

The measurements of the galactic disc infrared emission are summarized. The empirical models made to explain the spatial distribution of the infrared fluxes emitted by the galactic disc are recalled. The dust models are reviewed. Special emphasis is given to the difficulties in accounting for dust models. The proposition of a new component of the interstellar matter, the Polycyclic Aromatic Molecules (PAHs) is discussed. The results obtained using balloon-borne instruments are reported. The identification of the galactic interstellar very small grains to PAH molecules is strengthened. The present vision of the interstellar medium is described.