Gravitational waves (GWs) have a very weak absorption cross section. This makes them difficult to observe on the one hand, but it  is also for this reason they carry pristine information from their generation mechanism. Their observation may allow  glimpses  back to the highest redshifts and energies in the Universe. 

After a brief introduction to the significance of the observation of a stochastic gravitational wave background, I shall  concentrate on two possibilities: Generating gravitational waves from violent processes in the post-inflationary Universe and generating them during inflation. I shall show that in both cases different universal GW spectra are generated. I shall also discuss what we may learn from their observation.