AbstractIn large regions of densities and temperatures the properties of plasmas are influenced by Coulombic forces. Here especially hydrogen and helium plasmas are studied. The region of nonideality is given and the main effects as screening, quantum corrections, level shifts and the cut off for the sum over states are discussed. Based on methods of classical and quantum statistics corrections to the ideal EOS are calculated. Interpolational expressions covering the whole density-temperature

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... ane are given and phase transitions to highly ionized states are discussed. Level shifts due to nonideality are derived. Finally the ionization equilibrium and kinetics are considered theoretically focussing the attention on nonideality effects.