  In low-mass disks, turbulent torques are probably the most important way of
redistributing angular momentum. Here we present the theory of turbulent
accretion disks. We show the molecular viscosity is far too small to account
for the evolutionary timescale of disks, and we describe how turbulence may
result in enhanced transport of (angular) momentum. We then turn to the
magnetorotational instability, which thus far is the only mechanism that has
been shown to initiate and sustain turbulence in disks. Finally, we present
both the basis and the structure of alpha disk models.
