  Ultra-high energy cosmic rays (UHECRs) are the highest energy messengers of
the present universe, with energies up to $10^{20}$ eV. Studies of
astrophysical particles (nuclei, electrons, neutrinos and photons) at their
highest observed energies have implications for fundamental physics as well as
astrophysics. The primary particles interact in the atmosphere and generate
extensive air showers. Analysis of those showers enables one not only to
estimate the energy, direction and most probable mass of the primary cosmic
particles, but also to obtain information about the properties of their
hadronic interactions at an energy more than one order of magnitude above that
accessible with the current highest energy human-made accelerator. In this
contribution we will review the state-of-the-art in UHECRs detection. We will
present the leading experiments Pierre Auger Observatory and Telescope Array
and discuss the cosmic ray energy spectrum, searches for directional
anisotropy, studies of mass composition, the determination of the number of
shower muons (which is sensitive to the shower hadronic interactions) and the
proton-air cross section.
