The scanning tunneling microscope (STM) has permitted to reveal the Kondo structure of many molecules adsorbed on metallic surfaces.The Kondo effect is a complex many-body process in which the magnetic moment of a magnetic impurity in contact with an electron gas is completely screened to form the electronic ground state of the full system.The low-energy excitations of the system are of magnetic origin.These excitations are many,due to the electronic continuum furnished by the metal substrate, but they are concentrated in a small energy scale:the Kondo scale.The Kondo scale thus corresponds to the energy needed to destroy the ground state.