On the transition from Galactic to extragalactic cosmic rays

The Earth is permanently exposed to a flux of high-energy ionized nuclei - the cosmic rays. Most of these particles are accelerated in our Galaxy, most probably in supernova remnants. Cosmic rays are magnetically bound to our Galaxy up to energies of about $10^{17}$ to $10^{18}$ eV. At higher energies the observed particles most likely originate in other galaxies and are usually referred to as extra-galactic cosmic rays. Thus, the energy region between $10^{17}$ and $10^{18}$ eV is of great astrophysical interest. From a detailed measurement of the composition of cosmic rays in this energy regime we expect deeper insight into both, the origin of Galactic and extra-galactic cosmic rays. The status of the actual research will be reviewed and implications on our understanding of the origin of cosmic rays will be discussed.