Transition from Galactic to Extra-Galactic Cosmic Rays
  In this paper we review the main features of the observed Cosmic Rays spectrum in the energy range $10^{17} {\rm eV}~\div~10^{20} {\rm eV}$. We present a theoretical model that explains the main observed features of the spectrum, namely the second Knee and Dip, and implies a transition from Galactic to Extra-Galactic cosmic rays at energy $E\simeq 10^{18}$ eV, with a proton dominated Extra-Galactic spectrum. 
Roberto Aloisio