Possible sources for the gravitational wave background

The detection of gravitational waves from the coalescence of black holes has opened a new window for astronomy. Besides individual mergers, one can study the stochastic gravitational-wave background, i.e. the sum of all gravitational waves arriving at Earth, which are not from resolved sources. In this talk I will give an overview of the current predictions for this background, over a range of frequencies -- from binary neutron stars at 100 Hz to the mergers of super-massive black holes at 10^(-8) Hz, and even further to primordial gravitational waves generated during inflation. Of these, none have so far been detected, save for a signal consistent with a background from super-massive black hole coalescences. I will touch on how background sources are modelled, and on how these can be used to extend our understanding of physics.