Shakura and Sunyaev (1973) proposed turbulence in the gas as the source of an increased viscosity. Assuming subsonic turbulence and the disk height as an upper limit for the size of the eddies, the disk viscosity can be estimated as ν ν = α α c s H where c s is the sound speed, H is the scale height of the disk, and α α is a free parameter between zero (no accretion) and approximately one. In a turbulent medium ν ν ≈ ≈ v t u r b l t u r b , where v t u r b is the velocity of turbulent cells relative to the mean gas motion, and l t u r b is the size of the largest turbulent cells, which is estimated as l t u r b ≈ ≈ H = c s / Ω Ω and v t u r b ≈ ≈ c s , where Ω Ω = ( G M ) 1 / 2 r − − 3 / 2 is the Keplerian orbital angular velocity, r is the radial distance from the central object of mass M . By using the equation of hydrostatic equilibrium, combined with conservation of angular momentum and assuming that the disk is thin, the equations of disk structure may be solved in terms of the α α parameter. Many of the observables depend only weakly on α α , so this theory is predictive even though it has a free parameter.