  The focus of this Chapter is on describing the prospective sources of the
gravitational wave universe accessible to present and future observations, from
kHz, to mHz down to nano-Hz frequencies. The multi-frequency gravitational wave
universe gives a deep view into the cosmos, inaccessible otherwise. It has as
main actors core-collapsing massive stars, neutron stars, coalescing compact
object binaries of different flavours and stellar origin, coalescing massive
black hole binaries, extreme mass ratio inspirals, and possibly the very early
universe itself. Here, we highlight the science aims and describe the
gravitational wave signals expected from the sources and the information
gathered in it. We show that the observation of gravitational wave sources will
play a transformative role in our understanding of the processes ruling the
formation and evolution of stars and black holes, galaxy clustering and
evolution, the nature of the strong forces in neutron star interiors, and the
most mysterious interaction of Nature: gravity. The discovery, by the LIGO
Scientific Collaboration and Virgo Collaboration, of the first source of
gravitational waves from the cosmos GW150914, and the superb technological
achievement of the space mission LISA Pathfinder herald the beginning of the
new phase of exploration of the universe.
