The present status of our knowledge about the highest-energy particles in Nature

In the era of the multi-messenger astronomy, ultra-high energy cosmic rays offer the unique opportunity to investigate the nature of astrophysical sources and of particle interactions in an energy range far beyond that covered by current particle accelerators.
The Pierre Auger Observatory, the world’s largest cosmic ray detector, combines in a hybrid design the information from fluorescence telescopes, observing the longitudinal profile of extensive air showers, with a surface array, measuring the lateral distributions of secondary particles at the ground.
A review of selected results will be presented, focusing on the energy spectrum, nuclear mass composition measurements and anisotropies.
To connect plausibles UHECR sources to data at Earth, two possible astrophysical scenarios will be detailed: the acceleration and propagation of cosmic rays from steady or transient sources.
During the investigation, understanding of the astrophysical environment, such as in the case of starburst galaxies or galaxy clusters, will also be curciale to complete the multi-messenger scenario.