  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.
