Primordial Magnetic Fields and Their Development

In this talk I discuss the non-linear development of magnetic fields in the early universe. I very briefly mention a number of particle physics mechanisms for generating magnetic fields in the early universe, but the emphasis is on a review of the field theoretic aspects of the developments of these fields, mainly on the occurence of inverse cascades, i.e. generation of order from disorder in the sense of transfer of energy from smaller to larger scales. In this connection it is also briefly reviewed how the Silk effect (photon diffusion) is counteracted by the inverse cascade.