  In this article I review the main theoretical problems that are posed by the
highest energy end of the observed cosmic ray spectrum, stressing the
importance of establishing their composition in order to decide between
proposed scenarios. I then discuss the possibilities that are opened by the
detection of inclined showers with extensive air shower arrays. Recent progress
in modelling magnetic deviations for these showers has allowed the analysis of
inclined showers that were detected by the Haverah Park experiment. This
analysis disfavours models that predict a large proportion of photons in the
highest energy cosmic rays and open up new possibilities for future shower
array detectors particularly those, like the Pierre Auger Observatory, using
water Cherenkov detectors.
