  This review discusses some of the observational constraints on what we know
about the mass loss experienced by stars in the Asymptotic Giant Branch (AGB)
phase of evolution. Mass loss affects the maximum mass attained by the core of
an AGB star and hence its fate as a white dwarf or potentially a supernova. The
way mass loss depends on stellar initial parameters and time affects the yield
from AGB stars, in terms of elemental abundances and types of dust. The role of
pulsation, dust formation, chromospheres and other mechanisms which may
contribute to mass loss are assessed against observational evidence, and
suggestions are made for observations which could force significant new
progress in this field in the first decades of the 21st century. A better
understanding of AGB mass loss may be gained from a combination with studies of
first ascent red giant branch (RGB) stars and red supergiants, through
population studies and in different environments.
