The mass composition of cosmic rays above 10¹7 eV

It is shown that our knowledge of the mass composition of cosmic rays is deficient at all energies above 10^17 eV. 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 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 and interpret data on the arrival direction distribution of cosmic rays.