The intensity of cosmic rays is steeply falling with energy. The differential spectrum can be approximated by an E^−α power law. It shows some distinct features. The change in the spectral index α from 2.7 to 3.1 occurring at 3 × 10¹⁵ eV is called the knee. Another spectral break occurs at 3 × 10¹⁸ eV, the ankle, where the spectrum turns up again. The highest energies observed so far reach 3 × 10²⁰ eV. This observation is a major puzzle, and the subject is the focus of intense research.

Particles beyond the ankle are thought to be of extragalactic origin because their Lamor radius is larger than the size of the galaxy. Due to interactions with the 3K cosmic microwave background photons a sharp cut-off is expected at about 5 × 10¹⁹ eV, named after Greisen, Zatsepin and Kuz'min (GZK). The existence of the GZK cut-off in contrast to a continuing spectrum is currently under investigation. Different models for the origin of the ultra-high-energy events have been designed. Some of them are able to reproduce observed features, some of them are highly speculative, and all are awaiting some experimental validation. The nature, origin and propagation of cosmic rays are still a mystery 90 years after their discovery by Hess and Kohlhörster.

This paper focuses on the experimental situation regarding cosmic rays at the highest energies. More than 60 years after Pierre Auger initially recorded and correctly interpreted extended air showers, this field is more active than ever before. Existing data from the experiments performed so far will be reviewed and put into context. Current challenges will be discussed with the perspectives coming from new projects, in particular from the Pierre Auger Observatory in Argentina.