Accretion disk turbulence is primarily MHD turbulence, not hydrodynamical. The paper discusses the nature of turbulence in accretion disks, comparing it to planar Couette flow. It shows that only disks with constant angular momentum are unstable to nonlinear disturbances and develop enhanced turbulent transport. Hydrodynamical Keplerian disks are stable to nonlinear disturbances. The key to disk turbulence is the interaction between the stress tensor and the mean flow gradients. The weak field MHD instability, which is of great astrophysical importance, displays the same type of stress tensor – mean flow coupling that all classical local shear instabilities exhibit. Hydrodynamical Keplerian disks, on the other hand, do not. Accretion disk turbulence is MHD turbulence. The paper presents numerical simulations showing that nonlinear instabilities are present in disks with constant angular momentum (q=2), but not in Keplerian disks (q=3/2). The results indicate that the presence of a magnetic field is crucial for the development of turbulence in accretion disks. The magnetic field introduces angular velocity coupling to the stress tensor, enabling sustained outward turbulent transport. The paper also discusses the energy transport in magnetic disks, showing that the magnetic field plays a key role in the energy dynamics of the disk. The paper concludes that the onset of turbulence in disks is now well understood, and that the body of laboratory and numerical simulation experiments point to a consistent physical picture. Turbulence in Keplerian accretion disks resulting in enhanced viscous transport must be MHD turbulence. The study of accretion disk turbulence can now be approached at a level of detail comparable to what is possible in stellar convection studies. The subject is immature, and still of a form amenable to simple, key ideas. It is not obvious that everything that is obvious has already been done.Accretion disk turbulence is primarily MHD turbulence, not hydrodynamical. The paper discusses the nature of turbulence in accretion disks, comparing it to planar Couette flow. It shows that only disks with constant angular momentum are unstable to nonlinear disturbances and develop enhanced turbulent transport. Hydrodynamical Keplerian disks are stable to nonlinear disturbances. The key to disk turbulence is the interaction between the stress tensor and the mean flow gradients. The weak field MHD instability, which is of great astrophysical importance, displays the same type of stress tensor – mean flow coupling that all classical local shear instabilities exhibit. Hydrodynamical Keplerian disks, on the other hand, do not. Accretion disk turbulence is MHD turbulence. The paper presents numerical simulations showing that nonlinear instabilities are present in disks with constant angular momentum (q=2), but not in Keplerian disks (q=3/2). The results indicate that the presence of a magnetic field is crucial for the development of turbulence in accretion disks. The magnetic field introduces angular velocity coupling to the stress tensor, enabling sustained outward turbulent transport. The paper also discusses the energy transport in magnetic disks, showing that the magnetic field plays a key role in the energy dynamics of the disk. The paper concludes that the onset of turbulence in disks is now well understood, and that the body of laboratory and numerical simulation experiments point to a consistent physical picture. Turbulence in Keplerian accretion disks resulting in enhanced viscous transport must be MHD turbulence. The study of accretion disk turbulence can now be approached at a level of detail comparable to what is possible in stellar convection studies. The subject is immature, and still of a form amenable to simple, key ideas. It is not obvious that everything that is obvious has already been done.