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Intense mass loss occurs for low- and intermediate-mass stars on the asymptotic giant branch (AGB), and for the higher mass (≳8 M⊙) stars during their red supergiant evolution. These winds affect the evolution of the stars profoundly, creates circumstellar envelopes of gas and dust, as well as enrich the interstellar medium with heavy elements and grain particles. The mass loss characteristics are well-studied, but the basic processes are still not understood in detail, and the mass-loss rate of an individual star cannot be derived from first principles. These objects also provide us with fascinating systems, in which intricate interplays between various physical and chemical processes take place, and their relative simplicity in terms of geometry, density distribution, and kinematics makes them excellent astrophysical laboratories. The review concentrates on the aspects of AGB stars and their mass loss which are of particular interest in connection with ALMA.

This review presents the physical and chemical structure of circumstellar envelopes (CSEs) around stars on, and beyond, the asymptotic giant branch (AGB). It focusses on information that is relevant to the interpretation of the molecular line emission from these envelopes. The mass loss rate estimate from CO radio line emission is briefly discussed. It continues with an overview of the morphology and kinematics of AGB and post-AGB CSEs, based on the observational evidence obtained from molecular line emission. Also the small-scale structure of the circumstellar medium is discussed. Finally, a possible scenario for the puzzling, drastic change (at least apparently) from largely spherical AGB-CSEs to the prominent equatorial density enhancements that dominate in CSEs around late post-AGB objects, e.g., planetary nebulae (PNe), is put forward.