  The solar wind and the interstellar medium are permeated by large-scale
magnetic fields that render magnetohydrodynamic (MHD) turbulence anisotropic.
In the weak-turbulence limit in which three-wave interactions dominate,
analytical and high-resolution numerical results based on random scattering of
shear-Alfv\'en waves propagating parallel to a large-scale magnetic field, as
well as direct simulations demonstrate rigorously an anisotropic energy
spectrum that scales as $k^{-2}_\perp$, instead of the famous
Iroshnikov-Kraichnan (IK) spectrum of $k^{-3/2}$ for the isotropic case. Even
in the absence of a background magnetic field, anisotropy is found to develop
with respect to the local magnetic field, although the energy spectrum is
globally isotropic and is found to be consistent with a $k^{-3/2}$ scaling. It
is also found in direct numerical simulations that the energy cascade rate is
much closer to IK scaling than a Kolmogorov scaling. Recent observations in the
solar wind on cascade rates (as functions of the proton temperature and solar
wind speed at 1 AU) seem to support this result [Vasquez et al. 2007].
