At frequencies well below the proton (or ion) cyclotron frequency, the electric component of a plasma wave in a magnetic field is not important, and the magnetic component dominates physical processes. The physics of such a plasma are called hydromagnetics, and are usually well described by the MHD (magnetohydrodynamic) equations. Although hydromagnetic processes tend to proceed slowly as their typical saturation levels are quite high, they can play dominating roles in many cases. Hydromagnetic has been widely applied to various problems in the fields of nuclear fusion, space physics, and astrophysics.