  We describe recent work focused towards a better understanding of red
supergiant stars using 3D radiative-hydrodynamics (RHD) simulations with
CO5BOLD. A small number of simulations now exist that span up to seven years of
stellar time, at various numerical resolutions. Our discussion concentrates on
interferometric and spectroscopic observations. We point out a number of
problems, in particular the line depth and line width that are not well
reproduced by simulations. The most recent introduction of a non-grey treatment
of the radiation field dramatically improved the match with observations,
without solving all difficulties. We also review the newly revived effective
temperature scale controversy, and argue that it will only be solved using 3D
RHD models.
