  The Meccano of heavy fermion systems is shown on different cases going from
anomalous monochalcogenides to cerium intermetallic compounds with special
focus on the ideal case of the CeRu2Si2 series. Discussion is made in the frame
of the interplay between valence, electronic structure (Fermi surface), and
magnetism. The nice tools given by the temperature, the pressure, and the
magnetic field allow to explore different ground states as well as the slow
downhill ''race'' before reaching a Fermi liquid finish line at very low
temperature. Experimentally, the Gr"uneisen parameter i.e. the ratio of the
thermal expansion by the specific heat is a coloured magic number; its
temperature, pressure, and magnetic field dependence is a deep disclosure of
competing hierarchies and the conversion of this adaptive matter to external
responses.
