  We review the anomalous properties of heavy fermion compounds like
CeCu$_{6-x}$Au$_x$ or CeMIn$_5$ close to a zero temperature phase transition
called a quantum critical point. Anomalous behavior of the resistivity,
specific heat and magnetic properties observed in various compounds suggests a
fundamental breakdown of the Fermi liquid theory. Recent measurements on
YbRh$_2$Si$_2$, field-tuned through the quantum critical point, provide
interesting insights on the evolution of the Fermi liquid close to criticality.
We discuss the possibility of a kind of spin-charge separation at the quantum
critical point and make some remarks about its possible link with the
underscreened Kondo model.
