  High resolution maps of maser emission provide very detailed information on
processes occurring in circumstellar envelopes of late-type stars. A
particularly detailed picture of the innermost shells around AGB stars is
provided by SiO masers. Considerable progress is being made to provide
astrometrically aligned multi-transition simultaneous observations of these
masers, which are needed to better constrain the models. In view of the large
amount of high quality data available, models should now be developed to fully
explain all maser characteristics together (spatial distribution, variability,
etc). New generation instruments (VERA, VSOP-2), new observational techniques
(frequency-phase transfer), and new models promise important improvements of
our knowledge on this topic.
