Propagation of UHECRs
  In this general introduction to Ultra High Energy Cosmic Rays (UHECRs) we discuss the propagation of UHE protons and the GZK feature that is expected approaching $10^{20}\eV$ for homogeneously distributed sources. We also briefly present the effects of the propagation on other particles that can play the role of UHECRs. With the help of numerical simulations for the propgation of UHECRs, we show that the GZK feature cannot be accurately determined with the small sample of events with energies $\sim10^{20}\eV$ detected thus far by the largest two experiments, AGASA and HiRes. 
Daniel De Marco (Bartol Research Institute)