  Emission and absorption line observations of molecules in late-type stars are
a vital component in our understanding of stellar evolution, dust formation and
mass loss in these objects. The molecular composition of the gas in the
circumstellar envelopes of AGB stars reflects chemical processes in gas whose
properties are strong functions of radius with density and temperature varying
by more than ten and two orders of magnitude, respectively. In addition, the
interstellar UV field plays a critical role in determining not only molecular
abundances but also their radial distributions. In this article, I shall
briefly review some recent successful approaches to describing chemistry in
both the inner and outer envelopes and outline areas of challenge for the
future.
