  In the radio regime the mass-loss rate of AGB stars is best probed using
molecular (and atomic) line emission arising in the CSE formed by the stellar
wind. The numerical modelling of the circumstellar emission where intricate
interplays between physical and chemical processes take place, is a challenge.
The derived mass-loss rates depend crucially on the assumptions in the
circumstellar model, of which some can be constrained if enough observational
data exist. Therefore, a reliable mass-loss-rate determination for an
individual star requires, in addition to a detailed radiative transfer
analysis, good observational constraints in the form of multi-line observations
and radial brightness distributions. Of the methods used to estimate mass-loss
rates from galactic AGB stars those based on radiative transfer modelling of CO
line emission are most commonly used and possibly also the most accurate.
Typically, CO multi-transitional observations can constrain the mass-loss rate
to better than 50%, within the adopted circumstellar model. Comparison with
complementary methods, such as estimates based on dust radiative transfer
modelling coupled with a dynamical model, are consistent within a factor of
three.
