  Our knowledge of the mass composition of cosmic rays is deficient at all
energies above 10^17. Here systematic differences between different
measurements are discussed and, in particular, it is argued that there is no
compelling evidence to support the common assumption that vast majority of the
cosmic rays of the highest energies are protons. Our knowledge of the mass
needs to be improved if we are to resolve uncertainties about the energy
spectrum. Improvement is also needed for proper interpretation of data on the
arrival direction distribution of cosmic ray.
