  Although cosmic rays were discovered over 100 years ago their origin remains
uncertain. They have an energy spectrum that extends from 1 GeV to beyond 1020
eV, where the rate is less than 1 particle per km2 per century. Shortly after
the discovery of the cosmic microwave background in 1965, it was pointed out
that the spectrum of cosmic rays should steepen fairly abruptly above about 4 x
1019 eV, provided the sources are distributed uniformly throughout the
Universe. This prediction, by Greisen and by Zatsepin and Kuzmin, has become
known as the GZK-effect and in this article I discuss the current position with
regard to experimental data on the energy spectrum of the highest cosmic-ray
energies that have been accumulated in a search that has lasted nearly 50
years. Although there is now little doubt that a suppression of the spectrum
exists near the energy predicted, it is by no means certain that this is a
manifestation of the GZK-effect as it might be that this energy is also close
to the maximum to which sources can accelerate particles, with the
highest-energy beam containing a large fraction of nuclei heavier than protons.
The way forward is briefly mentioned.
