This chapter will introduce the theory and analytical methods of thermodynamics. Four postulates, fundamental equations, Euler equation, Gibbs-Duhem equation, entropy maximum principle, energy minimum principle, Legendre transformation, thermodynamic potentials and the corresponding minimum principles, Maxwell relations, thermodynamic stability, Phase change and Clapeyron equation, explicit expressions of chemical potentials, and dependence of boiling and freezing points on solute mole fraction are introduced in this chapter.