Abstract:
In recent years there has been a change of the paradigm for interpreting ultrahigh-energy cosmic rays (UHECRs). Data of the Pierre Auger Observatory and Telescope Array have revealed a number of unexpected features. In addition to the surprisingly small anisotropy of the arrival direction distribution, there is a non-trivial change of mass composition of cosmic rays. In this talk an overview of key results of UHECR measurements is given. Possible interpretations of the data are discussed, emphasizing the importance of our current understanding of extensive air showers and related uncertainties. Plans for addressing the remaining uncertainties in both measuring the cosmic ray composition and simulating extensive air showers will be presented in an outlook.