If charged particles move through the interplanetary or interstellar medium, they interact with a large-scale magnetic field such as the magnetic field of the Sun or the Galactic magnetic field. As these background fields are usually nearly constant in time and space, they can be approximated by a homogeneous field. If there are no additional fields, the particle trajectory is a perfect helix along which the par- cle moves at a constant speed. In reality, however, there are turbulent electric and magneticfields dueto the interstellaror solar wind plasma. These fields lead to sc- tering of the cosmic rays parallel and perpendicular to the background field. These scattering effects, which usually are of diffusive nature, can be described by s- tial diffusion coefficients or, alternatively, by mean free paths. The knowledge of these parameters is essential for describing cosmic ray propagation as well as d- fusive shock acceleration. The latter process is responsible for the high cosmic ray energies that have been observed. The layout of this book is as follows. In Chap. 1, the general physical scenario is presented. We discuss fundamental processes such as cosmic ray propagation and acceleration in different systems such as the solar system or the interst- lar space. These processes are a consequence of the interaction between charged cosmic particles and an astrophysical plasma (turbulence). The properties of such plasmas are therefore the subject of Chap. 2.