This chapter includes the following topics: State of a System Fundamental Equation for the Internal Energy Maxwell Equations Gibbs-Duhem Equation and the Phase Rule Legendre Transforms for the Definition of Additional Thermodynamic Potentials Thermodynamic Potentials for a Single-Phase System with One Species Other Kinds of Work Calculation of Thermodynamic Properties of a Monatomic Ideal Gas from Derivatives of a Thermodynamic Potential