Magnetic fields not only provide a basic tool for observing galaxies but constitute an important clue to interpret many dynamic processes taking place in them, mainly in spiral and irregular galaxies. As in the Sun, magnetic fields become important where gravity becomes lower, i.e. in the outermost regions. Any peripheric feature should be explained taking into account magnetic fields, in particular warps, truncations of stellar systems, lopsidedness and so on. Of special cosmological interest is the role of magnetic fields in the rotation curve of spirals. All these questions are still insufficiently explored and the study of magnetic fields in the whole universe open wide areas of interpretation.