Аннотация:Observational data and theoretical results on the large-scale magnetic fields in spiral galaxies are reviewed. Observed variations of position angle of polarized radio emission over a galaxy image are discussed. Observational data for our Galaxy are insufficient to decide whether the galactic magnetic field has circular or bisymmetric structure. Estimates and numerical calculations show that bisymmetric magnetic structures observed in many galaxies are the result of excitation of the lowest non axisymmetric magnetic field mode by the turbulent dynamo. The growth time for bisymmetric field in M51 is 300 million yr. In contrast, all non axisymmetric modes decay in M31, in accordance with observations. A source of galactic seed field due to superposition of magnetic loops ejected by supernovae is proposed.