  We review numerical simulations of MHD turbulence. The last decade has
witnessed fundamental advances both in the technical capabilities of direct
numerical simulation, and in our understanding of key physical processes.
Magnetic fields tap directly into the free energy sources in a sufficiently
ionized gas. The result is that adverse angular velocity and adverse
temperature gradients, not the classical angular momentum and entropy
gradients, destabilize laminar and stratified flow. This has profound
consequences for astrophysical accretion flows, and has opened the door to a
new era of numerical simulation experiments.}
