The historical and theoretical background for the different mathematical fo
rms commonly used to represent equations of state (EOS) for solids is revie
wed and some criteria are discussed which allow to select some specific for
ms, which are not only convenient mathematical forms but bring out the phys
ical meaning of the data. Some examples are discussed with emphasis on rece
nt experimental and theoretical data including noble gas solids, simple met
als and a few metals with special "anomalies" to illustrate the differences
involved in the use of different EOS forms and to point out the specific p
hysical reasons for differences in specific EOS forms.