In this paper, we briefly review some recent de Haas-van Alphen experiments on two contrasting types of heavy fermion/Kondo systems for varying concentrations x of the impurity ion, Ce or U. In both systems the Fermi surface is observed to vary continuously with x: in Ce x La 1 - x B 6 , the Ce ions behave as Kondo impurities while in U x Th 1 - x Be 1 3 they do not. In both systems, the f electrons are also found to contribute only partially to the Fermi surface volume, therefore raising questions concerning the applicability of Fermi liquid theory.