Open questions with ultra-high energy cosmic rays
  We briefly discuss three aspects related to the origin of ultra-high energy cosmic rays (UHECRs) namely: 1) particle acceleration in astrophysical sources; 2) transition to an extragalactic origin; 3) spectrum and anisotropies at the highest energies. 
Pasquale Blasi (1) ((1) INAF/Arcetri Astrophysical Observatory)