The observational evidence of large-scale magnetism is briefly reviewed. After accounting for the theoretical speculations on the origin of large-scale magnetic fields, it will be argued that CMB physics is probably the only framework where a primordial magnetic field could either be ruled out, or, more optimistically, ruled in. If a stochastic magnetic field is present prior to equality, accurate calculations of the (magnetized) CMB anisotropies are then required. The main idea is to combine plasma physics techniques and general relativistic techniques. Recent semi-analytical and numerical results of this approach will be described in the framework of a dedicated Boltzmann solver and in the light of the WMAP 5-year data.