Circumstellar envelopes of cool giants and supergiants contain atomic and molecular gas, and dust. The charateristic spectral features of these different components can be observed at optical (atoms), at radio (molecules) and at infrared wavelengths (dust). Since the detection of circumstellar matter around giants and supergiants most studies concentrated on detemining mass loss rates from observations of a single component assuming steady mass loss during late stellar evolution. Nevertheless, the IRAS colour colour diagramme and evolutionary models rather point to a non steady evolution during the mass loss phase of the star.