Variability is a characteristic feature of most AGB stars.
 Unfortunately, spectroscopic time series covering many of the
 relevant molecular bands and the dust emission simultaneously do not
 exist. Because of the different sensitivity of various observable
 features to different parameters and components of the atmosphere/
 envelope of an AGB variable (LPV) and because of the strong and time
 dependent interplay of pulsation, dust and molecules, such
 simultaneous and homogeneous data are the only way for a complete
 understanding of the physical processes involved. From the
 theoretical side, many of the tools for a quantitative interpretation
 of such complete spectroscopic time series are available now.
 The SWS on board of ISO covers the wavelength region where continuous
 dust emission starts to dominate and most of the important features
 in the spectra of LPVs (molecular bands of CO, SiO, H2O, HCN, C2H2 and
 C3, the solid state features of Silicates and SiC). While some of the
 mentioned bands and features can be observed from the ground, this
 requires different instruments and often excellent observing
 conditions. Other bands accesible with ISO are in spectral regions
 where the Earth's atmosphere is completely opaque. Only ISO can
 provide, probably for a long time, complete, homogeneous
 and simultaneous data of the molecular features and the dust emission
 in the atmospheres and envelopes of LPVs. Such data will be a unique
 opportunity to test and improve the time dependent models. We thus
 propose periodic SWS observations of a few selected 'typical' objects.