The physical ideas behind renormalization are just as relevant in cases – such as condensed matter physics – where the analogous higherorder corrections are infinite, though possibly large. In quantum mechanics, infinite momentum corresponds to zero distance, and our fields are certainly ‘point-like’. In the case of electrons in a metal, for example, it is surprising that the presence of the lattice ions, and the attendant band structure, affect the response of conduction electrons to external fields, so that their apparent inertia changes. Accepting the general framework of quantum field theory, then, the first thing we must obviously do is to modify the theory in some way so that integrals such as do not actually diverge, so that we can at least discuss finite rather than infinite quantities. This step is called ‘regularization’ of the theory.