The fundamental constants in physics and their time dependence

We discuss the fundamental constants in the Standard Model of particle physics. Furthermore we discuss possible changes of these constants on the cosmological time scale. The Grand Unification of the observed strong, electromagnetic and weak interactions implies relations between the time variations of the finestructure constant α and of the QCD scale Λ_c. A change of α by 10⁻¹⁵/year, as seen by an astrophysics experiment, implies a time variation of Λ_c of at least 10⁻¹⁵/year. An experiment in Quantum Optics, which was designed to look for a time variation of Λ_c, is discussed.