  We derive a weak turbulence formalism for incompressible MHD. Three-wave
interactions lead to a system of kinetic equations for the spectral densities
of energy and helicity. We find energy spectra solution of the kinetic
equations. The constants of the spectra are computed exactly and found to
depend on the amount of correlation between the velocity and the magnetic
field. Comparison with several numerical simulations and models is also made.
