  Ubiquity of unconventional phenomena observed in a series of heavy fermion
metals is discussed on the basis of an idea of critical valence fluctuations.
After surveying experimental aspects of these unconventional behaviors in
prototypical compounds, CeCu_2(Si,Ge)_2, under pressure, we propose that sharp
valence crossover phenomena are realized in CeCu_6, CeRhIn_5, and Ce(Ir,Rh)Si_3
by tuning the pressure and the magnetic field simultaneously, on the basis of
previous results for an extended Anderson lattice model with the Coulomb
repulsion U_fc between localized f-electron and itinerant conduction electrons.
