  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.
