The equation of state of astrophysical plasmas is, for a wide range of stars, nearly ideal with only small non-ideal Coulomb corrections. Calculating the equation of state of an ionizing plasma from a ground state ion, ideal gas model is easy, whereas, fundamental methods to include the small Coulomb corrections are difficult. Attempts to include excited bound states are also complicated by many-body effects that weaken and broaden these states. Nevertheless, the high quality of current observations, particularly seismic data, dictates that the best possible models should be used. The equation of state used in the OPAL opacity tables is based on many-body quantum statistical methods (Rogers, 1994; 1986; 1981) and is suitable for the modeling of seismic data. Extensive tables of the OPAL equation of state are now available. These tables cover the temperature range 5 × 10−3 to 1 × 108 K, the density range 10-14 to 105 g/cm3, the hydrogen mass fraction (X) range 0.0 to 0.8, and the metallicity (Z) range 0.0 to 0.04.