  We discuss and illustrate contributions that optical interferometry has made
on our current understanding of cool evolved stars. We include red giant branch
(RGB) stars, asymptotic giant branch (AGB) stars, and red supergiants (RSGs).
Studies using optical interferometry from visual to mid-infrared wavelengths
have greatly increased our knowledge of their atmospheres, extended molecular
shells, dust formation, and winds. These processes and the morphology of the
circumstellar environment are important for the further evolution of these
stars toward planetary nebulae (PNe) and core-collapse supernovae (SNe), and
for the return of material to the interstellar medium.
