Ultra High Energy Cosmic Rays and the Highest Energies Universe

We will discuss the main relevant aspects of the physics of ultra high energy cosmic rays. After a short recap of the experimental evidences, we will review theoretical models aiming at describing the sources of these extremely energetic particles opening a window on the highest energies universe. We will discuss the production of secondary particles and the possible tests of new physics that ultra high energy cosmic rays could provide. The present proceedings paper is mainly based on review papers written by the author in the last two years.