  MHD turbulence consists of waves that propagate along magnetic fieldlines, in
both directions. When two oppositely directed waves collide, they distort each
other, without changing their respective energies. In weak MHD turbulence, a
given wave suffers many collisions before cascading. "Imbalance" means that
more energy is going in one direction than the other. In general, MHD
turbulence is imbalanced. A number of complications arise for the imbalanced
cascade that are unimportant for the balanced one.
  We solve weak MHD turbulence that is imbalanced. Of crucial importance is
that the energies going in both directions are forced to equalize at the
dissipation scale. We call this the "pinning" of the energy spectra. It affects
the entire inertial range.
  Weak MHD turbulence is particularly interesting because perturbation theory
is applicable. Hence it can be described with a simple kinetic equation.
Galtier et al. (2000) derived this kinetic equation. We present a simpler, more
physical derivation, based on the picture of colliding wavepackets. In the
process, we clarify the role of the zero-frequency mode. We also explain why
Goldreich & Sridhar claimed that perturbation theory is inapplicable, and why
this claim is wrong. (Our "weak" is equivalent to Goldreich & Sridhar's
"intermediate.")
  We perform numerical simulations of the kinetic equation to verify our
claims. We construct simplified model equations that illustrate the main
effects. Finally, we show that a large magnetic Prandtl number does not have a
significant effect, and that hyperviscosity leads to a pronounced bottleneck
effect.
