Everyone knows what a metal is and can describe many of its characteristics. It is safe to say, however, that few people would define a metal as "a solid with a Fermi surface". This may nevertheless be the most meaningful definition of a metal that one can give today; it represents a profound advance in the understanding of why metals behave as they do. The concept of the Fermi surface, developed by quantum physics, provides a precise explanation of the main physical properties of metals: their conduction of electricity and heat, their hardness and ductility, their lustrous appearance and so on.