  In many evolutionary stages, low- and intermediate-mass stars show signs of
mixing of the surface material with material from the interior. To account for
all the details revealed by the observations it is necessary to include
non-standard physical processes in the models (e.g. atomic diffusion and
rotation-induced mixing). The study of mixing in stars of different masses,
ages, and chemical composition helps to identify and constrain these processes.
In this sense, stars in open clusters are the ideal targets. All stars in one
given cluster have the same age and chemical composition, and their masses can
be well estimated. By studying many clusters, one can separate and trace the
effects of these different parameters.
