The paper by Balbus and Hawley discusses the nature of turbulence in accretion disks, comparing it to planar Couette flow, a classical system prone to nonlinear shear instability. The authors find that only constant angular momentum disks are unstable to nonlinear disturbances and develop enhanced turbulent transport, while convective instabilities do not lead to enhanced transport. Hydrodynamical Keplerian disks are stable to nonlinear disturbances, and the key to disk turbulence lies in the interaction between the stress tensor and mean flow gradients. The weak field MHD instability, which is crucial in astrophysics, exhibits the same stress tensor-mean flow coupling as classical shear instabilities. However, hydrodynamical Keplerian disks do not exhibit this coupling. Accretion disk turbulence is MHD turbulence, and the presence of a magnetic field introduces angular velocity coupling to the stress tensor, enabling sustained outward turbulent transport. The authors conclude that many features of the onset of turbulence in disks are well understood, and the study of accretion disk turbulence can now be approached with detailed physical insights, similar to stellar convection studies.The paper by Balbus and Hawley discusses the nature of turbulence in accretion disks, comparing it to planar Couette flow, a classical system prone to nonlinear shear instability. The authors find that only constant angular momentum disks are unstable to nonlinear disturbances and develop enhanced turbulent transport, while convective instabilities do not lead to enhanced transport. Hydrodynamical Keplerian disks are stable to nonlinear disturbances, and the key to disk turbulence lies in the interaction between the stress tensor and mean flow gradients. The weak field MHD instability, which is crucial in astrophysics, exhibits the same stress tensor-mean flow coupling as classical shear instabilities. However, hydrodynamical Keplerian disks do not exhibit this coupling. Accretion disk turbulence is MHD turbulence, and the presence of a magnetic field introduces angular velocity coupling to the stress tensor, enabling sustained outward turbulent transport. The authors conclude that many features of the onset of turbulence in disks are well understood, and the study of accretion disk turbulence can now be approached with detailed physical insights, similar to stellar convection studies.