Introduction

Magnetic fields are present in almost every place in the Universe. Most of the luminous matter is tightly coupled to magnetic fields. Large-scale fields intersperse the gas in the Milky Way, in galaxies and in galaxy clusters. They contribute to the nonlinear interplay of turbulent motions in the interstellar and intracluster medium. The magnetic energy content affects the evolution of galaxies and galaxy clusters, contributes significantly to the total pressure of interstellar gas, is essential for the onset of star formation, and controls the density and distribution of cosmic rays in the interstellar medium (ISM). In spite of their importance, the evolution, structure and origin of magnetic fields are still open problems in fundamental physics and astrophysics.