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Methods of investigating interstellar magnetic fields in spiral galaxies are reviewed. Linearly polarized radio continuum emission yields information about strength and structure of the field. Data are available for our Galaxy and several nearby galaxies. The field strengths vary between 3 and 13 , Â¿G; a correlation with the average surface mass density of the galaxies is indicated. The field of M31 is probably concentrated in a ring-like torus, as predicted by the dynamo theory. The field structure of M33, however, appears to be bisymmetric as expected in the primordial theory of field origin. Other candidates for a bisymmetric field structure are M51, M81, M83, NGC 2903, and NGC 6946, but the observations are not yet conclusive. The magnetic field in IC 342 is too complicated to be described in terms of a simple model. The interaction processes between the field and the various components of interstellar gas should be investigated in more detail.