The mass composition of cosmic rays above 10¹7 eV

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.