  Cool evolved stars are the main source of chemical enrichment of the
interstellar medium. Understanding their mass loss offers a unique opportunity
to study the cycle of matter. We discuss interferometric studies and their
comparison to latest state-of-the-art dynamic model atmospheres. They show
broad agreement for asymptotic giant branch stars. For red supergiants,
however, current models cannot explain observed extensions by far, pointing to
missing physical processes in their models, and uncertainties in our general
understanding of mass loss. We present ongoing imaging and time-series
observations that may provide the strongest constraint and may help to identify
missing dynamic processes. VLTI studies will remain the highest spatial
resolution observations at ESO into the ELT era, complemented by ALMA
observations. We discuss crucial improvements in both instrumental and
operational areas for surface imaging of cool evolved stars in the era of the
ELT.
