The evidence is reviewed for a universal magnetic field of strength 10−9–10−8G; it has been extended to include the diffuse fields of galactic clusters and the extensive magnetic halos of spiral galaxies. Some likely effects of the universal fieldB 0 are as follows: (1) As suggested previously,B 0 is coupled to protogalaxies and evolves into magnetic structures which depend on the angle between the field and the gas rotational axis. These provide the blueprints for the various types of the Hubble sequence, (ii) The relatively few ‘grand-design’ spiral galaxies result from tidal interactioon (M51-type), but ‘normal’ spirals form as a result of the spiral oblique field) magnetic blueprint acting on sheared gravitational instabilities (Goldreich and Lynden-Bell). (iii) The model explains the prevalent warped galactic disks and perhaps their flat H1 rotation curves. (iv) A variety of puzzling H1 concentrations may have hydromagnetic explanations; they include the high-velocity clouds, streamers, rings and central systems. (v) Clusters of galaxies are known to have diffuse magnetic fields, and these are likely to explain the absence of spiral galaxies and the nature of the intracluster gas. (vi) Spiral galaxies are now known to have extensive magnetic halos. These appear explicable only in terms of the universal magnetic field model.