Model equations of state determining the thermodynamic characteristics of matter in various states of aggregation are reviewed. Methods for describing the thermodynamics of gases, liquids, and plasmas are described. Quantum-mechanical models for solids and a quasiclassical model of matter are discussed. Models which can be used to study melting, evaporation, structural and electronic phase transitions in solids, and phase transitions in nonideal plasmas are also discussed. The ranges of applicability of the various methods are determined. The results of model-based calculations are compared with the experimental data available.