  Extragalactic cosmic ray protons with an injection spectrum of the type
$E^{-2.7}$ show a spectrum on earth with a dip due to the Bethe-Heitler pair
production against the photons of the cosmic microwave background. The dip is
produced in the energy region $10^{18} - 4\times 10^{19}$ eV with position and
shape that reproduce with high accuracy the spectrum observed experimentally.
This interpretation of the observed data predicts the existence of an energy
scale that signals a possible transition from galactic to extragalactic cosmic
rays. In fact, at energies lower than a characteristic value $E_c\approx
1\times 10^{18}$ eV, determined by the equality between the rate of energy
losses due to pair production and adiabatic losses, the spectrum of cosmic rays
flattens in all cases of interest. In this model, the transition from galactic
to extragalactic cosmic rays occurs at some energy below $E_c$, corresponding
to the position of the so-called second knee. Another viable explanation of the
observed data is based on a completely different approach assuming a mixed
composition with protons and nuclei at energies $E\ge 10^{19}$ eV. This
scenario implies a transition from galactic to extragalactic cosmic rays at the
ankle energies ($\sim 10^{19}$ eV). In the present paper we will review the
main features of the dip model comparing it with the model of transition at the
ankle.
