When looking at matter in the universe neutral atoms, molecules and solids constitute an exception. Most of the matter that we can observe resides in hot stellar environments and is ionized. Here, electrons are partially or fully detached from the atomic nuclei. Such a gas of freely moving electrons and ions is called a "plasma". On earth high energy concentrations are required to ionize large amounts of matter and thereby turn it into a plasma. The best known natural and man-made examples are lightnings and electric discharges, respectively. High-power lasers made a new efficient way of creating plasma in the laboratory necessary.

Unlike from a gas of neutral particles (e.g. air) electrons and ions are charged and interact via the Coulomb force. This force has such a long range that many electrons and ions feel each other and therefore move collectively. Collective oscillations of electrons against the ions at the plasma frequency are a major mode of motion in a plasma. The plasma frequency depends on the electron density and is higher than the frequency of visible light in dense plasmas. In this case, light cannot penetrate into the plasma, but is reflected at the surface. This happens, for example, in a normal mirror where free electrons in metallic aluminum form a dense plasma oscillating with a period of about 300 attoseconds, i.e. five to ten times faster than visible light.

In a typical plasma positive and negative charges occur in equal numbers such that the plasma as a whole is neutral. Each attempt to separate the charges is counteracted by huge electric fields and can normally be achieved over short distances only. However, the new lasers which are now being developed are strong enough to separate sizeable amounts of charge creating electric fields in the order of TV/m. These laser-induced plasma fields exceed the fields used in conventional particle accelerators by many orders of magnitude and afford promise for building compact laser-driven accelerators for a broad range of applications. Insights into dynamics of ultrahigh-density plasmas will be instrumental in realizing inertial fusion for developing environmentally friendly nuclear fusion reactors.