  I review recent work on the radial transport of angular momentum in ionized,
Keplerian accretion disks. Proposed mechanisms include hydrodynamic and MHD
local instabilities and long range effects mediated by wave transport. The most
promising models incorporate the Velikhov-Chandrasekhar instability, caused by
an instability of the magnetic field embedded in a differentially rotating
disk. This has the important feature that the induced turbulent motions
necessarily transport angular momentum outward. By contrast, convective modes
may transport angular momentum in either direction. Combining the magnetic
field instability with an $\alpha-\Omega$ dynamo driven by internal waves leads
to a model in which the dimensionless viscosity scales as $(H/r)^{4/3}$.
However, this model has a phenomenology which is quite different from the
$\alpha$ disk model. For example, an active disk implies some source of
excitation for the internal waves. In binary systems with a mass ratio of order
unity the most likely exciting mechanism is a parametric instability due to
tidal forces. This implies that in systems where the accretion stream is
intermittent, like MV Lyrae or TT Ari, epochs when the mass flow is absent or
very small will be epochs in which the disk shrinks and becomes relatively
inactive and dark. This model also implies that forced vertical mixing is
important, even in convectively stable disks. I discuss various observational
tests of this model and the focus of current theoretical work.
