  Stellar evolution computations provide the foundation of several methods
applied to study the evolutionary properties of stars and stellar populations,
both Galactic and extragalactic. The accuracy of the results obtained with
these techniques is linked to the accuracy of the stellar models, and in this
context the correct treatment of the transport of chemical elements is crucial.
Unfortunately, in many respects calculations of the evolution of the chemical
abundance profiles in stars are still affected by sometime sizable
uncertainties. Here, we review the various mechanisms of element transport
included in the current generation of stellar evolution calculations, how they
are implemented, the free parameters and uncertainties involved, the impact on
the models, and the observational constraints.
