Properties of High-Density Matter in Intense Magnetic Fields

We summarize recent results on the properties of matter in intense magnetic fields which are believed to exist in gravitationally collapsed bodies. The equations of state of a Fermi gas in magnetic fields are presented; they are superpositions of quantities relevant to a one-dimensional gas if the density is low enough or the field strong enough. For laboratory matter, the transition field is approximately ${10}^9$ G.