  The GIM Mechanism was introduced by Sheldon L. Glashow, John Iliopoulos and
Luciano Maiani in 1970, to explain the suppression of Delta S=1, 2 neutral
current processes and is an important element of the unified theories of the
weak and electromagnetic interactions. Origin, predictions and uses of the GIM
Mechanism are illustrated. Flavor changing neutral current processes (FCNC)
represent today an important benchmark for the Standard Theory and give strong
limitations to theories that go beyond ST in the few TeV region. Ideas on the
ways constraints on FCNC may be imposed are briefly described.
