  The magnetohydrodynamical behavior (MHD) of accretion disks is reviewed. A
detailed presentation of the fundamental MHD equations appropriate for
protostellar disks is given. The combination of a weak (subthermal) magnetic
field and Keplerian rotation is unstable to the magnetorotational instability
(MRI), if the degree of ionization in the disk is sufficiently high. The MRI
produces enhanced angular momentum and leads to a breakdown of laminar flow
into turbulence. If the turbulent energy is dissipated locally, standard
"$\alpha$" modeling should give a reasonable estimate for the disk structure.
Because away from the central star the ionization fraction of protostellar
disks is small, they are generally not in the regime of near perfect
conductivity. Nonideal MHD effects are important. Of these, Ohmic dissipation
and Hall electromotive forces are the most important. The presence of dust is
also critical, as small interstellar scale grains absorb free charges that are
needed for good magnetic coupling. On scales of AU's there may be a region near
the disk midplane that is magnetically decoupled, a so-called {\em dead zone.}
But the growth and settling of the grains as time evolves reduces their
efficiency to absorb charge. With ionization provided by coronal X-rays from
the central star (and possibly also cosmic rays), protostellar disks may be
sufficiently magnetized throughout most of their lives to be MRI active,
especially away from the disk midplane.
