The strength of the total magnetic field in our Milky Way from radio synchrotron measurements is about 6 μG (0.6 nT), averaged over a radius of about 1 kpc around the Sun. Diffuse polarized radio emission and Faraday rotation of the polarized emission from pulsars and background sources show many small-scale magnetic features, but the overall field structure in our Galaxy is still under debate. – In nearby galaxies, radio synchrotron observations reveal dynamically important magnetic fields of 10–30 μG (1–3 nT) total strength in the spiral arms. Fields with random orientations are concentrated in spiral arms, while ordered fields (observed in radio polarization) are strongest in interarm regions and follow the orientation of the adjacent gas spiral arms. Faraday rotation of the diffuse polarized radio emission from the disks of spiral galaxies sometimes reveals large-scale patterns which are signatures of regular fields generated by dynamos, but in most galaxies the field structure is more complicated. – Strong magnetic fields are also observed in radio halos around edge-on galaxies, out to large distances from the plane. The ordered halo fields usually form an X-shaped pattern. Diffuse polarized radio emission in the outer disks and halos is an excellent tracer of galaxy interactions and ram pressure by the intergalactic medium.