  Thin, Keplerian accretion disks generically become gravitationally unstable
at large radius. I investigate the nonlinear outcome of such instability in
cool disks using razor-thin, local, numerical models. Cooling, characterized by
a constant cooling time t_c, drives the instability. I show analytically that,
if the disk can reach a steady state in which heating by dissipation of
turbulence balances cooling, then the dimensionless angular momentum flux
density \alpha = ((9/4) \gamma (\gamma-1) \Omega t_c)^{-1}. Numerical
experiments show that: (1) if t_c \gtrsim 3\Omega^{-1} then the disk reaches a
steady, gravito-turbulent state in which Q \sim 1 and cooling is balanced by
heating due to dissipation of turbulence; (2) if t_c \lesssim 3\Omega^{-1},
then the disk fragments, possibly forming planets or stars; (3) in a steady,
gravito-turbulent state, surface density structures have a characteristic
physical scale \sim 64 G \Sigma/\Omega^2 that is independent of the size of the
computational domain.
