In this chapter, basic thermodynamic framework, necessary for understanding of hydrothermal ore deposits, is presented. Historically speaking, chemical thermodynamics evolved as a branch of science with a classical approach that deals with reversible processes. There is also a non-classical branch, comprising irreversible thermodynamics, which essentially is a contemporary development. This chapter deals with some of the integral properties of chemical thermodynamics along with the most pertinent expressions and derivation of equations of state (EOS) for pure mineral phases involving Gibbs free energy (G), enthalpy (H), entropy (S), volume (V), and heat capacity at constant pressure (Cp) at equilibrium pressure-temperature. Furthermore, this chapter also includes energetics of mineral solid solutions, thermodynamic properties of fluids, pertinent to hydrothermal ore systems, apart from other related aspects. In the end, solved numerical problems, along with self-assessment questions (SAQ) are provided for the readers to practice and explore more. There are excellent text books and study materials available on mineralogical/geochemical thermodynamics (e.g., Nordstrom & Munoz, 1985; Chatterjee, 1991; Anderson, 2017).