Various approaches for the representation of equation of state data for sol
ids under strong compression are discussed. The theoretical background for
reasonable extrapolations to higher pressures and higher as well as lower t
emperatures is described. The distinction between ideal, regular, and anoma
lous behaviour allows to gain deeper insight into the electronic changes oc
curring in various solids under strong compression. The discussion of exper
imental data for various regular solids leads finally to an estimate of the
accuracy obtained in the present realisation of a practical pressure scale
based on equation of state measurements.