The outer photospheres and inner regions of the circumstellar shells
of AGB stars, both "O rich" and "C rich", will be explored by
studies at relatively high spectral resolution of a number of molecular
bands and forbidden atomic lines. The aim is to establish more reliable
abundance criteria for these regions and gain further insight into
their physical and chemical conditions. This will also result in
more accurate estimates of mass loss rates from CO line observations
at millimetre wavelengths, as well as deeper understanding concerning
the mass loss mechanisms.