  Gravitational waves (GW) can constitute a unique probe of the primordial
universe. In many cases, the characteristic frequency of the emitted GW is
directly related to the energy scale at which the GW source is operating in the
early universe. Consequently, different GW detectors can probe different energy
scales in the evolution of the universe. After a general introduction on the
properties of a GW stochastic background of primordial origin, some examples of
cosmological sources are presented, which may lead to observable GW signals.
