  Heavy fermion compounds are complex systems but excellent materials to study
quantum criticality with the switch of different ground states. Here a special
attention is given on the interplay between magnetic and valence instabilities
which can be crossed or approached by tuning the system by pressure or magnetic
field. By contrast to conventional rare earth magnetism or classical s wave
superconductivity, strong couplings may occur with drastic changes in spin or
charge dynamics. Measurements on Ce materials give already a sound basis with
clear key factors; they have pointed out that close to a magnetic or a valence
criticality unexpected phenomena such as unconventional superconductivity, non
Fermi liquid behaviour and the possibility of re-entrance phenomena under
magnetic field. Recent progresses in the growth of Yb heavy fermion compounds
give the perspectives of clear interplays between valence and magnetic
fluctuations and also the possibility to enter in new situations such as
valence transitions inside a sole doublet crystal field ground state.
