Last June several Pulsar Timing Array (PTA) collaborations reported evidence for a signal compatible with a gravitational wave (GW) origin. Although the significance did not yet cross the psychological '5sigma' threshold, we are likely witnessing the opening of a new window on the Universe: the nano-Hz GW window. I will discuss the principles of PTAs, the most likely origin of the observed signal and its implications for astrophysics and cosmology.