This lecture discusses itinerant magnetism, focusing on the importance of on-site repulsion U and the concept of spontaneous band splitting. The Stoner criterion and the pressure-induced collapse of ferromagnetism in nickel are also explored, along with field-induced ferromagnetism and the non-Fermi liquid behavior in metallic ferromagnetism. Examples of non-magnetic ferromagnets and the marginal breakdown of the Fermi-liquid state are presented, highlighting the spontaneous band polarization phenomenon.