  We analyze the decay laws of the kinetic and magnetic energies and the
evolution of correlation lengths in freely decaying incompressible
magnetohydrodynamic (MHD) turbulence.
  Scale invariance of MHD equations assures that, in the case of constant
dissipation parameters ({\it i.e.} kinematic viscosity and resistivity) and
null magnetic helicity, the kinetic and magnetic energies decay in time as $E
\sim t^{-1}$, and the correlation lengths evolve as $\xi \sim t^{1/2}$.
  In the helical case, assuming that the magnetic field evolve towards a
force-free state, we show that (in the limit of large magnetic Reynolds number)
the magnetic helicity remains constant, the kinetic and magnetic energies decay
as $E_v \sim t^{-1}$ and $E_B \sim t^{-1/2}$ respectively, while both the
kinetic and magnetic correlation lengths grow as $\xi \sim t^{1/2}$.
