Outstanding Problems in Particle Astrophysics
  The general features of the cosmic-ray spectrum have been known for a long time. Although the basic approaches to understanding cosmic-ray propagation and acceleration have also been well understood for many years, there are several questions of great interest that motivate the current intense experimental activity in the field. The purpose of this lecture is to introduce the main topics of the School and to relate the theoretical questions to the experiments that can answer them. 
Thomas K. Gaisser