  The nuclei fraction in cosmic rays (CR) far exceeds the fraction of other CR
species, such as antiprotons, electrons, and positrons. Thus the majority of
information obtained from CR studies is based on interpretation of isotopic
abundances using CR propagation models where the nuclear data and isotopic
production cross sections in p- and alpha-induced reactions are the key
elements. This paper presents an introduction to the astrophysics of CR and
diffuse gamma rays and discusses some of the puzzles that have emerged recently
due to more precise data and improved propagation models. Merging with
cosmology and particle physics, astrophysics of CR has become a very dynamic
field with a large potential of breakthrough and discoveries in the near
future. Exploiting the data collected by the CR experiments to the fullest
requires accurate nuclear cross sections.
