Twenty Years of UHECR Discoveries in the Era of Giant Arrays

The study of Ultra-High-Energy Cosmic Rays (UHECR) has dramatically changed in the last 20 years with the advent of the giant cosmic ray arrays - the Pierre Auger Observatory and the Telescope Array project. The larger and more sophisticated cosmic ray detectors are capable of making a variety of measurements on cosmic ray induced extensive air showers advancing our knowledge on the key questions of sources, spectrum, and composition of the cosmic rays. With the very large apertures of these experiments, we are now able to collect enough high energy data to show signs of anisotropy including a dipole and some medium scale sources. We have measured the cosmic ray energy spectrum in detail to show that there are several features that can be interpreted with underlying physics. Similarly, the composition of cosmic rays has been measured and shown to vary over energy range measured by these experiments adding to the interpretation of the sources and propagation of the cosmic rays. While significant advances have been made, we have yet remaining questions. The cosmic ray spectra are in remarkable agreement in observations from the north and south, with the exception of the energy of cutoff or suppression at the highest energy. Is this a difference a real physical difference? Meanwhile, the highest energy cosmic rays, E > 10^20 eV, which should be the least bent by magnetic fields and thus point back to sources appear to be isotropic. This presentation will discuss the present state of the field and future prospects.