  In the first step, experiments on a single cerium or ytterbium Kondo impurity
reveal the importance of the Kondo temperature by comparison to other type of
couplings like the hyperfine interaction, the crystal field and the intersite
coupling. The extension to a lattice is discussed. Emphasis is given on the
fact that the occupation number $n_f$ of the trivalent configuration may be the
implicit key variable even for the Kondo lattice. Three $(P, H, T)$ phase
diagrams are discussed: CeRu$_2$Si$_2$, CeRhIn$_5$ and SmS.
