The origin of ultra-high energy cosmic rays (UHECRs) is a long-standing mystery in astrophysics. Two collaborations are running experiments, the Telescope Array in the northern hemisphere and the Pierre Auger Observatory in the southern hemisphere, recording the signals coming from the extensive air showers made by the most energetic particles observed in nature, in search of their origin. We review the issues raised by the experimental results and discuss the possible origins of UHECR.