  Pressure is the cleanest way to tune heavy fermion systems to a quantum phase
transition in order to study the rich physics and competing phases, and the
comparison between ytterbium and cerium systems is particularly fruitful. We
briefly review the mechanisms in play and show some examples of expected and
unexpected behaviour. We emphasize the importance of the valence changes under
pressure and show how modern synchrotron techniques can accurately determine
this, including at low temperature.
