One of the fundamental problems in theoretical astrophysics is the origin of turbulence that facilitates the angular momentum transport in accretion disks.  The magneto-rotational instability (MRI) is considered by many researchers to provide a viable answer to that problem.  The properties of small perturbations will be reviewed and a novel scenario of weakly nonlinear evolution of the MRI that is based on resonant as well as non-resonant three-wave interactions will be introduced and discussed. Such mechanism relies on the energy exchange between the MRI and other stable eigenmodes.  The resulting dynamical processes range from bounded irregular nonlinear oscillations of the MRI, through its nonlinear decay instability, to explosive instability of all the relevant perturbations. Triads of resonantly interacting waves thus serve as building blocks for a weakly nonlinear turbulence theory.