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Published online by Cambridge University Press: 12 April 2016
During the last two decades there has been a tremendous renewal of interest in the circumstellar environment of evolved stars. This renewal has been mainly driven by sensitive microwave observing techniques. Microwave spectroscopy has resulted in a number of fundamental discoveries in circumstellar shells, including the discovery of maser lines and complex circumstellar chemistry. Historically, interest in circumstellar shells originates from a desire to understand the mass-loss process. It has been known for at least 50 years that circumstellar gas is the origin of interstellar gas and dust. As such the circumstellar shell becomes the pathway through which elements created in the interior of the central star enrich galactic abundances. High resolution spatial and spectral techniques and an ever improving knowledge of stellar evolution has resulted in an increased but still incomplete understanding of the mass-loss process.