Convection is of major importance in both energy transport and mixing in
stars. Convective penetration, or overshooting from unstable into the
surrounding stable layers, extends the zone of effective mixing influencing
both the thermal structure and evolution of stars. We here consider a
range of approaches that can advance our understanding of this phenomenon:
the fitting of stellar models to clusters and binary systems, theoretical
studies, numerical simulation, and the new tools of helio- and
asteroseismology. Other mixing processes that may affect the evolution of
the sun and stars are also discussed