  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.
