  The problem of cosmic-ray scattering in the turbulent electromagnetic fields
of the interstellar medium and the solar wind is of great importance due to the
variety of applications of the resulting diffusion coefficients. Examples are
diffusive shock acceleration, cosmic-ray observations, and, in the solar
system, the propagation of coronal mass ejections. In recent years, it was
found that the simple diffusive motion that had been assumed for decades is
often in disagreement both with numerical and observational results. Here, an
overview is given of the interaction processes of cosmic rays and turbulent
electromagnetic fields. First, the formation of turbulent fields due to plasma
instabilities is treated, where especially the non-linear behavior of the
resulting unstable wave modes is discussed. Second, the analytical and the
numerical side of high-energy particle propagation will be reviewed by
presenting non-linear analytical theories and Monte-Carlo simulations. For the
example of the solar wind, the impact of anisotropic and dynamical turbulence
models will be discussed. In addition, it will be shown how further
complications can be treated that arise from the large-scale magnetic field
geometry and turbulent electric fields. The transport properties of energetic
particles can thus be calculated for current turbulence models so that they
withstand a comparison with measurements taken in the solar wind.
