The Theory of the Galactic Magnetic Field

We discuss the role of the magnetic field in determining the large scale structure and dynamics of the interstellar medium. We then discuss the origin and maintenance of the galactic field. The two major competing theories are that the field is primordial and connected to an intergalactic field or that the field is removed from and regenerated within the galaxy. We review the theoretical and observational basis for these theories. Finally, we discuss cosmic ray acceleration and confinement in the interstellar medium.