Stefan Westerhoff\uffff\uffff
Columbia University

The Highest Energy Cosmic Rays

Cosmic ray particles were discovered almost one hundred years ago, but
still very little is known about the origin of the most energetic
particles, above and around 1018 eV.  The existence of particles at
these energies, the highest energies observed in the Universe,
continues to challenge our imagination: where do they come from, how
are they accelerated, and how can they travel astronomical distances
without substantial loss of energy.

We are currently in an exciting new era in cosmic ray physics, with
instruments now producing data of unprecedented quality and quantity
to tackle the many open questions. Over the last 5 years, the High
Resolution Fly's Eye (HiRes) air fluorescence stereo detector has
accumulated data characterized by excellent angular resolution. The
world's largest detector for cosmic radiation, the Pierre Auger
Observatory in Argentina, is nearing completion, and first results
have already been published.

In this talk, I will review the current status of cosmic ray physics
and summarize recent results from the HiRes and Auger experiments on
the energy spectrum, composition, and arrival direction distribution
of ultra-high-energy cosmic rays.