There is little doubt that the magnetic field plays a major role in the formation, evolution and structure of many astrophysical and geophysical objects like galaxies, molecular clouds, accretion disks, stars and planets. As such, magnetic field has received considerable attention during the last decades. However in spite of these efforts, many questions remain largely open from astrophysical and geophysical points of view, on theoretical and numerical grounds. This is largely due to the inherent complexity of the non-linear magnetic processes as magnetohydrodynamic (MHD) turbulence and dynamo, to the uncertainties regarding the validity of the ideal MHD approximation and to the difficulty in computing MHD flows.